Manufacturing method of semiconductor device

ABSTRACT

A semiconductor device includes a first-conductivity-type semiconductor layer including an active region in which a transistor having impurity regions is formed and a marginal region surrounding the active region, a second-conductivity-type channel layer formed between the active region and the marginal region and forming a front surface of the semiconductor layer, at least one gate trench formed in the active region to extend from the front surface of the semiconductor layer through the channel layer, a gate insulation film formed on an inner surface of the gate trench, a gate electrode formed inside the gate insulation film in the gate trench, and at least one isolation trench arranged between the active region and the marginal region to surround the active region and extending from the front surface of the semiconductor layer through the channel layer, the isolation trench having a depth equal to that of the gate trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. application Ser. No. 14/301,521, filed onJun. 11, 2014, and allowed on Jul. 17, 2014, which was a Divisional ofU.S. application Ser. No. 13/491,581, filed on Jun. 7, 2012, and issuedas a U.S. Pat. No. 8,772,827 on Jul. 8, 2014, the subject matters ofwhich are incorporated herein by reference. The grandparent applicationSer. No. 13/491,581 is based upon and claims the benefit of priorityfrom Japanese Patent Applications No. 2011-128604 filed on Jun. 8, 2011,No. 2011-142148 filed on Jun. 27, 2011, and No. 2012-111746 filed on May15, 2012, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and amanufacturing method thereof. The semiconductor device includes a regionfor isolating an active region in which a transistor having a trenchgate structure is formed, and a marginal region which surrounds theouter periphery of the active region and defines an outer edge of thesemiconductor device.

BACKGROUND

In order to prevent mutual interference of a plurality of elementsformed on a semiconductor substrate, there is conventionally knowntechnology in which the elements adjoining each other are electricallyisolated.

For example, there is known a method that includes a step ofsequentially layering a silicon oxide film and a silicon nitride film ona silicon substrate and removing a portion of the silicon nitride filmby etching using a photoresist as a mask, a step of selectivelyoxidizing a portion of the silicon oxide film exposed by the removal ofthe silicon nitride film to thereby form a LOCOS oxide film in theexposed portion of the silicon oxide film, and a step of forming buriedoxide films on the silicon substrate at opposite sides of the LOCOSoxide film. In this method, an inter-element isolating region is made upof the LOCOS oxide film and the buried oxide films.

The above-described element isolating technology may also be used in,for example, electrically isolating an active region in which an elementis formed from a marginal area defining an outer edge of a semiconductordevice.

SUMMARY

A semiconductor device of the present disclosure includes: afirst-conductivity-type semiconductor layer including an active regionin which a transistor having a plurality of impurity regions is formedand a marginal region surrounding an outer periphery of the activeregion; a second-conductivity-type channel layer formed between theactive region and the marginal region so as to form a front surface ofthe semiconductor layer; at least one gate trench formed in the activeregion to extend from the front surface of the semiconductor layerthrough the channel layer; a gate insulation film formed on an innersurface of the at least one gate trench; a gate electrode formed insidethe gate insulation film in the at least one gate trench; and at leastone isolation trench arranged between the active region and the marginalregion to surround the outer periphery of the active region and formedto extend from the front surface of the semiconductor layer through thechannel layer, the at least one isolation trench having a depth equal toa depth of the at least one gate trench.

With this configuration, the channel layer extending between the activeregion and the marginal region is partitioned by the at least oneisolation trench, thereby providing insulation and isolation between theactive region and the marginal region. Accordingly, the channel layer ofthe active region can be electrically isolated from the channel layer ofthe marginal region.

While a method of electrically isolating the channel layer of an activeregion is conventionally available, the conventional method differs fromthe method of the present disclosure in which isolation trenches areformed in the semiconductor device. The conventional method suffers froma number of problems.

Specifically, in the conventional method, ion implantation for formingthe channel layer is performed after a LOCOS oxide film in the surfacelayer portion of the semiconductor layer or a recess LOCOS oxide film isformed by thermal oxidation. Therefore, even if ions are acceleratedtoward the entire area of the front surface of the semiconductor layer,the accelerated ions are blocked by the LOCOS oxide film in the surfacelayer portion or the recess LOCOS oxide film. Thus, implantation of theions toward the portions directly below the LOCOS oxide film in thesurface layer portion or the recess LOCOS oxide film is prevented.Accordingly, the channel layers are separately formed at the side of theactive region and the side of the marginal region with respect to theLOCOS oxide film enabling the channel layer of the active region to beelectrically isolated from the channel layer of the marginal region.

However, in the conventional method, the LOCOS oxide film needs to beformed relatively thick in order to adjust the electric fields generatedfrom the terminal of the transistor. For this reason, it is necessary toperform the heat treatment for a long period of time when forming theLOCOS oxide film, which may result in deterioration of the devicecharacteristics of the semiconductor device. Further, if the width ofthe LOCOS oxide film (the element isolation width) is too small, theactive region and the marginal region cannot be appropriately isolated.For this reason, a wide area is required for element isolation, whichmay result in a problem in that the size of the semiconductor devicegrows larger.

In contrast, with the semiconductor device of the present disclosure,the active region and the marginal region are electrically insulated andisolated from each other by the at least one isolation trench.Therefore, even if the width of the at least one isolation trench issmaller than the width of the LOCOS oxide film, the channel layers canbe completely divided. Accordingly, the size of the semiconductor devicecan be reduced.

The semiconductor device of the present disclosure can be manufacturedby a manufacturing method of a semiconductor device, which includes:forming a hard mask on a first-conductivity-type semiconductor layerhaving an active region and a marginal region surrounding an outerperiphery of the active region; simultaneously forming at least one gatetrench in the active region and at least one isolation trenchsurrounding the outer periphery of the active region between the activeregion and the marginal region, by selectively etching the semiconductorlayer from a front surface thereof using the hard mask; forming achannel layer extending between the active region and the marginalregion and divided by the at least one isolation trench between theactive region and the marginal region, by implanting asecond-conductivity-type impurity into a surface layer portion of thesemiconductor layer while keeping an entire area of the front surface ofthe semiconductor layer exposed; forming a gate insulation film on aninner surface of the at least one gate trench; forming a gate electrodeby filling an electrode material into the at least one gate trenchinside the gate insulation film; and forming a plurality of impurityregions forming a transistor in cooperation with the gate electrode inthe active region, by selectively implanting impurities into the surfacelayer portion of the semiconductor layer.

With this method, the at least one isolation trench is simultaneouslyformed with the at least one gate trench. Therefore, a manufacturingmethod of a semiconductor device can be simplified. Further, since it isnot necessary to perform thermal oxidation for a long period of time,unlike the case where a LOCOS oxide film is formed, devicecharacteristics (of the MOSFET) can be improved.

The semiconductor device of the present disclosure may further include:a trench insulation film formed on an inner surface of the at least oneisolation trench; and a buried electrode formed inside the trenchinsulation film in the at least one isolation trench.

With this configuration, a trench insulation film is formed on the innersurface of the at least one isolation trench and a buried electrode isfilled into the inside of the trench insulation film. Therefore, adepletion layer generated from the channel layer of the active regioncan be successfully widened toward the marginal region along thetransverse direction parallel to the front surface of the semiconductorlayer by the electric fields of the buried electrode. Accordingly, thevoltage resistance of the semiconductor device can be sufficientlysecured.

The trench insulation film and the buried electrode can be formed by themanufacturing method of a semiconductor device of the presentdisclosure, wherein said forming the gate insulation film includesforming a trench insulation film on an inner surface of the at least oneisolation trench, and wherein said forming the gate electrode includesforming a buried electrode by filling the electrode material into the atleast one isolation trench inside the trench insulation film.

With this method, the gate insulation film and the trench insulationfilm can be simultaneously formed, and the gate electrode and theimbedding electrode can also be simultaneously formed.

The semiconductor device of the present disclosure may further include:a first first-conductivity-type high-concentration impurity regionformed on a bottom surface of the at least one isolation trench. Animpurity concentration of the first high-concentration impurity regionmay be higher than an impurity concentration of the semiconductor layer.

With this configuration, the first high-concentration impurity region isformed in the bottom surface of the at least one isolation trench. Forthis reason, the depletion layer generated from the channel layer of theactive region can be widened in the transverse direction so as to avoidthe first high-concentration impurity region. Accordingly, electricfield concentration on the bottom surface of the at least one isolationtrench can be reduced.

The first high-concentration impurity region can be formed by themanufacturing method of a semiconductor device of the presentdisclosure, wherein the method may further include: forming, on a bottomsurface of the at least one isolation trench, a firstfirst-conductivity-type high-concentration impurity region having animpurity concentration higher than an impurity concentration of thesemiconductor layer, by implanting first-conductivity-type impuritiesinto the bottom surface of the at least one isolation trench.

The semiconductor device of the present disclosure may further include:an inter-layer insulation film formed on the semiconductor layer; afirst terminal formed on the inter-layer insulation film and connectedto the gate electrode through the inter-layer insulation film; and asecond terminal formed on the inter-layer insulation film and connectedto one of the impurity regions of the transistor through the inter-layerinsulation film. Further, the at least one isolation trench may bepositioned directly below the second terminal.

In this case, the second terminal may be connected through theinter-layer insulation film to the buried electrode of the at least oneisolation trench arranged directly below the second terminal.

With this configuration, the second terminal and the buried electrodecan be connected using plugs extending directly downward from the secondterminal through the inter-layer insulation film. Therefore, it is notnecessary to draw wiring lines from the buried electrode in a horizontaldirection, which may save space.

This structure can be formed by the manufacturing method of asemiconductor device of the present disclosure, wherein the method mayfurther include: forming an inter-layer insulation film on thesemiconductor layer, forming a first terminal on the inter-layerinsulation film, the first terminal being connected to the at least onegate electrode; and forming a second terminal on the inter-layerinsulation film, the second terminal being connected to one of theimpurity regions of the semiconductor layer and the buried electrode.

In this case, said forming the first terminal and said forming thesecond terminal may be performed simultaneously by depositing a metallicmaterial on the entire surface of the inter-layer insulation film andthen patterning the metallic material into a specified shape.

With this method, the first terminal and the second terminal can beformed in the same step. Therefore, the semiconductor can be more easilyand cost-effectively manufactured.

The number of the at least one isolation trench may be greater than two(plurality of isolation trenches) and the isolation trenches may differin perimeter from one another.

In this case, the second terminal may be connected to the buriedelectrode disposed closest to the active region among the buriedelectrodes formed in the plurality of isolation trenches. Further, awidth of the buried electrode connected to the second terminal may belarger than a width of the remaining buried electrodes not connected tothe second terminal.

With this configuration, an alignment margin when connecting the secondterminal to the buried electrode can be secured and the contact area ofthe buried electrode with respect to the second terminal can increase.Further, the width of the isolation trenches may be uniform.

A pitch of the isolation trenches may decrease as the isolation trenchesextend from the active region toward the marginal region or may increaseas the isolation trenches extend from the active region toward themarginal region. Further, the pitch of the isolation trenches may beuniform.

In particular, if the pitch of the isolation trenches decreases as theisolation trenches extend from the active region toward the marginalregion, the depletion layer generated from the channel layer of theactive region can be stably widened even in a position relativelydistant from the active region.

The pitch of the isolation trenches may be smaller than the pitch of thegate trench. For example, the pitch of the isolation trenches may rangefrom 0.3 μm to 5.0 μm. With this configuration, the voltage resistanceof the semiconductor device can increase.

The number of the at least one isolation trench may be one.

The semiconductor device of the present disclosure may further include:an inter-layer insulation film formed on the semiconductor layer; afirst terminal formed on the inter-layer insulation film and connectedto the gate electrode through the inter-layer insulation film; and asecond terminal formed on the inter-layer insulation film and connectedto one of the impurity regions of the transistor through the inter-layerinsulation film. In this case, the at least one isolation trench may bepositioned directly below the first terminal.

The semiconductor device of the present disclosure may further include:a second first-conductivity-type high-concentration impurity regionformed on a bottom surface of the at least one gate trench. In thiscase, an impurity concentration of the second high-concentrationimpurity region may be higher than an impurity concentration of thesemiconductor layer.

The second high-concentration impurity region can be simultaneouslyformed with the first high-concentration impurity region. In otherwords, the second high-concentration impurity region and the firsthigh-concentration impurity region can be formed by the manufacturingmethod of a semiconductor device of the present embodiment, wherein saidforming the first first-conductivity-type high-concentration impurityregion includes implanting the first-conductivity-type impurities usingthe hard mask and wherein the method further includes: forming,simultaneously with said forming the first first-conductivity-typehigh-concentration impurity region, a second first-conductivity-typehigh-concentration impurity region on a bottom surface of the at leastone gate trench, by implanting the first-conductivity-type impuritiesinto the bottom surface of the at least one gate trench exposed from thehard mask.

With this method, the first and the second high-concentration impurityregions can be formed by implanting the first-conductivity-typeimpurities using the hard mask used in forming the gate trench and theat least one isolation trench. Accordingly, the number of steps inmanufacturing a semiconductor device can be reduced.

The second terminal may be formed to cover the active region, and thefirst terminal may include a gate pad to which a bonding wire isconnected. The gate pad may surround an outer periphery of the secondterminal, and the at least one isolation trench may bypass the gate padwhen seen in a plan view.

With this configuration, the force transferred to the at least oneisolation trench when an impact is applied to the gate pad, for example,in the course of bonding wires to the gate pad, can be reduced.

In this case, the at least one isolation trench may be recessed towardthe active region with respect to the gate pad when seen in a plan viewsuch that the at least one isolation trench inwardly bypasses the gatepad or may protrude away from the active region with respect to the gatepad when seen in a plan view such that the at least one isolation trenchoutwardly bypasses the gate pad.

In the semiconductor device of the present disclosure, the impurityregions may include a MOS structure forming a trench-gate-type MOSFET(Metal Oxide Semiconductor Field Effect Transistor) in cooperation withthe gate electrode. In this case, the MOS structure may include: afirst-conductivity-type source region exposed on the front surface ofthe semiconductor layer and configured to form a portion of a sidesurface of the at least one gate trench; a second-conductivity-typechannel region formed at a side of a rear surface of the semiconductorlayer with respect to the source region so as to make contact with thesource region and configured to form a portion of the side surface ofthe at least one gate trench; and a first-conductivity-type drain regionformed at the side of the rear surface of the semiconductor layer withrespect to the channel region so as to make contact with the channelregion and configured to form a bottom surface of the at least one gatetrench.

With this configuration, the active region in which the trench-gate-typeMOSFET is formed can be electrically isolated from the marginal regionby the at least one isolation trench.

In this case, the second terminal may also include a source terminalconnected to the source region.

In the semiconductor device of the present disclosure, the impurityregions may include an IGBT structure making up a trench-gate-type IGBT(Insulated Gate Bipolar Transistor) in cooperation with the gateelectrode. In this case, the IGBT structure may include: afirst-conductivity-type emitter region exposed on the front surface ofthe semiconductor layer and configured to form a portion of a sidesurface of the at least one gate trench; a second-conductivity-type baseregion formed at a side of a rear surface of the semiconductor layerwith respect to the emitter region so as to make contact with theemitter region and configured to form a portion of the side surface ofthe at least one gate trench; and a first-conductivity-type collectorregion formed at the side of the rear surface of the semiconductor layerwith respect to the base region so as to make contact with the baseregion and configured to form a bottom surface of the at least one gatetrench.

With this configuration, the active region in which the trench-gate-typeIGBT is formed can be electrically isolated from the marginal region bythe at least one isolation trench.

In this case, the second terminal may also include an emitter terminalconnected to the emitter region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic plan view of a semiconductor deviceaccording to an embodiment of the present disclosure.

FIG. 2 illustrates a schematic partial section view of the semiconductordevice shown in FIG. 1.

FIG. 3A, which shows the same section view as in FIG. 2, illustrates astep of manufacturing the semiconductor device shown in FIG. 1.

FIG. 3B illustrates a step subsequent to the step shown in FIG. 3A.

FIG. 3C illustrates a step subsequent to the step shown in FIG. 3B.

FIG. 3D illustrates a step subsequent to the step shown in FIG. 3C.

FIG. 3E illustrates a step subsequent to the step shown in FIG. 3D.

FIG. 3F illustrates a step subsequent to the step shown in FIG. 3E.

FIG. 3G illustrates a step subsequent to the step shown in FIG. 3F.

FIG. 3H illustrates a step subsequent to the step shown in FIG. 3G.

FIG. 3I illustrates a step subsequent to the step shown in FIG. 3H.

FIG. 3J illustrates a step subsequent to the step shown in FIG. 3I.

FIG. 3K illustrates a step subsequent to the step shown in FIG. 3J.

FIG. 3L illustrates a step subsequent to the step shown in FIG. 3K.

FIG. 4 illustrates a first modified example of a high-concentrationimpurity region shown in FIG. 2.

FIG. 5A illustrates a second modified example of the high-concentrationimpurity region shown in FIG. 2.

FIG. 5B illustrates an enlarged view of a trench insulating filmindicated by a dot-line circle A in FIG. 5A.

FIG. 6 illustrates another example of an arrangement pattern of a trenchisolating region shown in FIG. 1.

FIG. 7, which shows the same section view as in FIG. 2, illustrates asemiconductor device according to another embodiment of the presentdisclosure (without having a wide trench).

FIG. 8 illustrates a modified example of a gate terminal shown in FIG.7.

FIG. 9, which shows the same section view as in FIG. 2, illustrates asemiconductor device according to another embodiment of the presentdisclosure (having a decreasing trench pitch).

FIG. 10, which shows the same section view as in FIG. 2, illustrates asemiconductor device according to another embodiment of the presentdisclosure (having an increasing trench pitch).

FIG. 11, which shows the same section view as in FIG. 2, illustrates asemiconductor device according to another embodiment of the presentdisclosure (having a single trench).

FIG. 12, which shows the same section view as in FIG. 2, illustrates asemiconductor device according to another embodiment of the presentdisclosure (including an IGBT).

FIG. 13 illustrates a graph showing the changes of a breakdown voltage(BVDSS) when the number and the pitch of the isolation trenches ischanged.

FIG. 14 illustrates a schematic plan view of a semiconductor deviceaccording to an embodiment of a reference example.

FIG. 15 illustrates a partially enlarged view of the semiconductordevice shown in FIG. 14.

FIG. 16A illustrates a schematic section view of the semiconductordevice taken along line A-A in FIG. 15, and FIG. 16B illustrates aschematic section view of the semiconductor device taken along line B-Bin FIG. 15.

FIG. 17 illustrates a circuit diagram for explaining an equivalentcircuit of the semiconductor device shown in FIG. 14.

FIGS. 18A and 18B, which show the same section views as in FIGS. 16A and16B, illustrate a step of manufacturing the semiconductor device shownin FIG. 14.

FIGS. 19A and 19B illustrate a step subsequent to the step shown inFIGS. 18A and 18B.

FIGS. 20A and 20B illustrate a step subsequent to the step shown inFIGS. 19A and 19B.

FIGS. 21A and 21B illustrate a step subsequent to the step shown inFIGS. 20A and 20B.

FIGS. 22A and 22B illustrate a step subsequent to the step shown inFIGS. 21A and 21B.

FIGS. 23A and 23B illustrate a step subsequent to the step shown inFIGS. 22A and 22B.

FIGS. 24A and 24B illustrate a step subsequent to the step shown inFIGS. 23A and 23B.

FIGS. 25A and 25B illustrate a step subsequent to the step shown inFIGS. 24A and 24B.

FIGS. 26A and 26B illustrate a step subsequent to the step shown inFIGS. 25A and 25B.

FIG. 27 illustrates a modified example of a double-ended zener diodeshown in FIG. 14.

FIG. 28 illustrates a first modified example of a source terminal and agate terminal when using the double-ended zener diode shown in FIG. 27.

FIG. 29 illustrates a second modified example of the source terminal andthe gate terminal when using the double-ended zener diode shown in FIG.27.

FIGS. 30A and 30B, which show the same section views as in FIGS. 16A and16B, illustrate schematic section views of a semiconductor deviceaccording to another embodiment of the reference example.

FIG. 31 illustrates a schematic plan view (partially enlarged view) of asemiconductor device according to another embodiment of the referenceexample.

FIG. 32A illustrates a schematic section view of the semiconductordevice taken along line C-C in FIG. 31, and FIG. 32B illustrates aschematic section view of the semiconductor device taken along line D-Din FIG. 31.

FIG. 33 illustrates a circuit diagram for explaining an equivalentcircuit of the semiconductor device shown in FIGS. 32A and 32B.

FIGS. 34A and 34B, which show the same section views as in FIGS. 32A and32B, illustrate schematic section views of a semiconductor deviceaccording to another embodiment of the reference example.

DETAILED DESCRIPTION

Reference will be made in detail to various embodiments, examples ofwhich are illustrated in the accompanying drawings. In the followingdetailed description, numerous specific details are set forth in orderto provide a thorough understanding of the present disclosure. However,it will be apparent to one of ordinary skill in the art that the presentdisclosure may be practiced without these specific details. In otherinstances, well-known methods, procedures, systems, and components havenot been described in detail so as to not unnecessarily obscure aspectsof the various embodiments.

<Overall Plan-View Configuration of Semiconductor Device>

FIG. 1 illustrates a schematic plan view of a semiconductor device 1according to an embodiment of the present disclosure.

The semiconductor device 1 has, e.g., a rectangular chip shape when seenin a plan view. The lengths of the chip-shaped semiconductor device 1 inthe up-down and the left-right directions in FIG. 1 on the sheet surfaceare approximately several millimeters each.

An electrode film 4 including a source terminal 2 and a gate terminal 3is formed on the front surface of the semiconductor device 1. Theelectrode film 4 is formed such that the source terminal 2 is surroundedby the rectangular ring-shaped gate terminal 3 extending along lateralend surfaces 5 of the semiconductor device 1. A gate pad 6 is installedin one corner portion of the gate terminal 3. Bonding wires 7 areconnected to the gate pad 6. In FIG. 1, the source terminal 2 isindicated transparently to show a plurality of unit cells 13 of a MOSFET14, to be described later.

Cut-out regions 8 are formed between the source terminal 2 and the gateterminal 3 and at the outer side of the gate terminal 3 (at the side ofthe lateral end surfaces 5 of the semiconductor device 1) (see thecross-hatched portions in FIG. 1). The cut-out regions 8 refer to theportions from which the electrode film 4 is removed to prevent thesource terminal 2 and the gate terminal 3 from making contact with eachother.

In the present embodiment, the cut-out region 8 disposed between thesource terminal 2 and the gate terminal 3 is formed into a rectangularshape extending along the lateral end surfaces 5 of the semiconductordevice 1 in which one corner portion of the cut-out region 8 is recessedtoward the source terminal 2 so as to inwardly bypass the gate pad 6.The gate pad 6 is installed in a pad space 9 of the gate terminal 3formed by the recessed portion of the cut-out region 8.

A ring-shaped trench isolation region 10 extending along the cut-outregion 8 is formed in the semiconductor device 1. A ring-shaped marginalregion 11 surrounding the outer periphery of the trench isolation region10 is formed in the portion between the trench isolation region 10 andthe lateral end surfaces 5 of the semiconductor device 1. An activeregion 12 defined by the trench isolation region 10 is formed at a sideopposite to the marginal region 11 with respect to the trench isolationregion 10.

Like the cut-out region 8 disposed between the source terminal 2 and thegate terminal 3, the trench isolation region 10 is formed so as toinwardly bypass the gate pad 6. Accordingly, a force transferred to thetrench isolation region 10 when an impact is applied to the gate pad 6,for example, in the course of bonding the wires 7 to the gate pad 6, canbe reduced.

The MOSFET 14, in which the plurality of unit cells 13 is arranged in alattice pattern, is formed in the active region 12. The arrangementpattern of the unit cells 13 is not limited to a lattice pattern but maybe, e.g., a stripe pattern or a zigzag pattern.

The source terminal 2 includes a source center portion 15 formeddirectly above the active region 12 so as to cover all of the unit cells13 arranged in a lattice pattern and a source extension portion 16extending from the source center portion 15 toward the trench isolationregion 10 so as to cover a part of a plurality of isolation trenches 39,to be described later. The source terminal 2 also includes groundcontacts 17 formed in the source extension portion 16. The groundcontacts 17 are connected to a wide electrode 49, to be described later.

Source pads 19, to which bonding wires 18 are connected, are installedin the source center portion 15. The source pads 19 are provided inplural numbers (for example, three, in the present embodiment) and arearranged side by side, e.g., from the position diagonal to the pad space9 along a peripheral edge of the source terminal 2.

The gate terminal 3 is formed directly above the marginal region 11 andis connected to a gate electrode 37 (to be described later) of theMOSFET 14 by a gate extension wiring line (not shown).

<Partial Cross-Section Configuration of Semiconductor Device 1>

FIG. 2 illustrates a schematic partial section view of the semiconductordevice 1 shown in FIG. 1.

As set forth above, the active region 12, the trench isolation region 10and the marginal region 11 are defined in the semiconductor device 1. Aselements shared by the regions 10 to 12, the semiconductor device 1includes a substrate 20 made of n⁺ type silicon (e.g., having aconcentration ranging from 1.0×10¹⁹ to 1.0×10²¹ atoms·cm⁻³, which shallapply hereinafter), an epitaxial layer 23 (semiconductor layer) made ofn⁻ type silicon (e.g., having a concentration ranging from 5.0×10¹⁶ to5.0×10⁴ atoms·cm⁻³, which shall apply hereinafter) and formed on a frontsurface 21 (upper surface) of the substrate 20, a p⁻ type channel layer26 (e.g., having a concentration ranging from 1.0×10¹⁵ to 1.0×10¹⁸atoms·cm⁻³, which shall apply hereinafter) formed along the entire frontsurface of the epitaxial layer 23 so as to form a front surface 24 ofthe epitaxial layer 23 and exposed from the lateral end surfaces 5 ofthe semiconductor device 1, and a drain layer 27 formed on the side ofthe substrate 20 with respect to the channel layer 26 so as to makecontact with the channel layer 26 and formed along the entire frontsurface 24 of the epitaxial layer 23 like the channel layer 26.

The thickness of the epitaxial layer 23 may range, e.g., from 3 μm to100 μm. The thickness of the channel layer 26 may range, e.g., from 0.3μm to 2.0 μm. N type impurities, e.g., phosphor (P) or arsenic (As), maybe included in the substrate 20 and the epitaxial layer 23. P typeimpurities, e.g., boron (B) or aluminum (Al), may be included in thechannel layer 26.

The MOSFET 14 formed in the active layer 12 is a trench gate type MOSFET(Metal Oxide Semiconductor Field Effect Transistor). In the epitaxiallayer 23, gate trenches 28 are formed in a lattice pattern so as toextend from the front surface 24 of the epitaxial layer 23 through thechannel layer 26. The bottom portion of each of the gate trenches 28reaches the drain layer 27.

In the epitaxial layer 23, the plurality of unit cells 13, each of whichhas a rectangular parallelepiped shape (a square shape when seen in aplan view), is formed in the respective window portions surrounded bythe lattice-like gate trenches 28.

The depth D₁ of the gate trenches 28 measured from the front surface 24of the epitaxial layer 23 may range, e.g., from 0.5 μm to 5.0 μm. Thewidth W₁ of the gate trenches 28 may range, e.g., from 0.10 μm to 1.0μm. The pitch P₁ of the gate trenches 28 may range, e.g., from 0.5 μm to5.0 μm.

In each of the unit cells 13, an n⁺ type source region 31 is formed inthe surface layer portion of the channel layer 26. The source region 31is formed in the surface layer portion of each of the unit cells 13 sothat the source region 31 can be exposed to the front surface 24 of theepitaxial layer 23 to form the front surface 24 and the source region 31can form the upper portion of the side surface of each of the gatetrenches 28. The thickness of the source region 31 in the direction fromthe front surface 24 toward the substrate 20 may range, e.g., from 0.1μm to 1.0 μm.

The source region 31 is not formed in the unit cells 13 (dummy cells 32)arranged at the outermost part of the active region 12 so as to form anouter edge of the lattice and not be given the function of the MOSFET14. In the dummy cells 32, the front surface 24 of the epitaxial layer23 is formed by the channel layer 26.

In the channel layer 26 of each of the unit cells 13, the portiondisposed at the side of the substrate 20 (at the side of the rearsurface 25 of the epitaxial layer 23) with respect to the source region31 is a channel region 33, in which the conductivity type of the channellayer 26 is maintained. The thickness of the channel region 33 mayrange, e.g., from 0.2 μm to 1.0 μm. The bottom portion of the channelregion 33 is positioned closer to the front surface 24 of the epitaxiallayer 23 than the bottom surface 30 of each of the gate trenches 28.

The region of the epitaxial layer 23 disposed at the side of thesubstrate 20 with respect to the channel region 33 is an n⁻ type drainregion 34, in which the state available after the epitaxial growth iskept as is. The drain region 34 is arranged at the side of the substrate20 with respect to the channel region 33 and makes contact with thechannel region 33.

A p⁺ type channel contact region 35 (e.g., having a concentrationranging from 1.0×10¹⁸ to 1.0×10²⁰ atoms·cm⁻³, which shall applyhereinafter) extending from the front surface 24 of the epitaxial layer23 to the channel region 33 through the source region 31 is formed inthe central portion of each of the unit cells 13.

A gate insulation film 36 made of silicon oxide (SiO₂) is formed on theinner surface of each of the gate trenches 28 so as to cover the entirearea of the inner surface of each of the gate trenches 28. The inside ofthe gate insulation film 36 is filled with polysilicon doped withimpurities, whereby the gate electrode 37 is buried in each of the gatetrenches 28.

The gate electrode 37 has an upper surface 38 flush with the frontsurface 24 of the epitaxial layer 23. The term “flush” in thisdescription means, for example, that the upper surface 38 of the gateelectrode 37 and the front surface 24 of the epitaxial layer 23 areformed on the same plane. However, the term “flush” conceptually mayalso include the case where a small depression is unintentionally formedin the course of forming the gate electrode 37.

The plurality of isolation trenches 39 dug from the front surface 24 ofthe epitaxial layer 23 toward the substrate 20 is formed in theepitaxial layer 23 of the trench isolation region 10. In order to reducethe size of the semiconductor device 1 while increasing the voltageresistance of the semiconductor device 1, the number of the isolationtrenches 39 may be set to, e.g., two through twenty. In someembodiments, the number of the isolation trenches 39 may be set to fivethrough fifteen. In other embodiments, the number of the isolationtrenches 39 may be set to ten through fifteen. In the presentembodiment, the number of the isolation trenches 39 is eight.

The isolation trenches 39 differ in perimeter from one another. Each ofthe isolation trenches 39 has a ring shape surrounding the MOSFET 14(the active region 12) and is arranged along the entire circumference ofthe trench isolation region 10. In the trench isolation region 10,therefore, the ring-shaped isolation trenches 39 and the ring-shapedchannel layers 26 divided in plural by the mutually-adjoining isolationtrenches 39 are arranged alternately.

The isolation trenches 39 include narrow trenches 42 having a width W₂equal to the width W₁ of the gate trenches 28 and a wide trench 43having a width W₂′ larger than the width W₁ of the gate trenches 28. Thewidth W₂′ of the wide trench 43 may be, for example, 1.2 to 5.0 times aslarge as the width W₁ of the gate trenches 28, and more particularly,may range from 0.12 μm to 5.0 μm.

In the present embodiment, the wide trench 43 is a trench disposed atthe innermost side (near the active region 12) of the trench isolationregion 10 among the isolation trenches 39. The wide trench 43 is formedto have a width such that an alignment margin when forming the groundcontacts 17, to be described later, can be secured and the contact areabetween the wide trench 43 and the ground contacts 17 can be increased.

The narrow trenches 42 are provided at a regular interval. The distancebetween the centers of the narrow trenches 42 adjoining each other (thetrench pitch P₂) may be smaller than the pitch P₁ of the gate trenches28. Alternatively, the pitch P₂ may be equal to or larger than the pitchP₁. The pitch P₂ of the narrow trenches 42 may range, e.g., from 0.3 μmto 5.0 μm.

The depth D₂ of the isolation trenches 39 measured from the frontsurface 24 of the epitaxial layer 23 may be equal to the depth D₁ of thegate trenches 28 (e.g., from 0.5 μm to 5.0 μm).

The width W₃ occupied by the isolation trenches 39 (the width of thetrench isolation region 10) may be, e.g., approximately 20 μm.

Below the isolation trenches 39, an n type high-concentration impurityregion 44 (a first high-concentration impurity region) having a higherimpurity concentration than that of the n⁻ type drain layer 27 is formedto be spaced apart from the channel layer 26 (so as not to contact withthe channel layer 26). The high-concentration impurity region 44 isformed to extend under the isolation trenches 39 adjoining each other.The high-concentration impurity region 44 forms the lower ends of bottomsurfaces 41 and side surfaces 40 of the isolation trenches 39.

A trench insulation film 45 made of silicon oxide (SiO₂) is formed onthe inner surface of each of the isolation trenches 39 so as to coverthe entire area of the inner surface of each of the isolation trenches39.

The inside of the trench insulation film 45 is filled with polysilicondoped with impurities, whereby buried electrodes 46 (narrow electrodes48 and the wide electrode 49) are buried in the isolation trenches 39.The buried electrodes 46 have upper surfaces 47 flush with the frontsurface 24 of the epitaxial layer 23.

An inter-layer insulation film 50 made of SiO₂ is formed on theepitaxial layer 23.

In the inter-layer insulation film 50, contact holes 51 and 52 extendingthrough the inter-layer insulation film 50 in the thickness directionare formed directly above the source region 31 of each of the unit cells13 and the wide electrode 49, respectively.

On the inner surfaces of the contact holes 51 and 52, for example,Ti/TiN barrier films are formed. The inside of each of the barrier filmsis filled with tungsten (W), whereby a plug-like contact (a sourcecontact 53 or the ground contact 17) is formed.

On the front surface 24 of the epitaxial layer 23, only the bottomsurface of the source contact 53 is connected to the source region 31.On the other hand, the ground contact 17 protrudes toward the bottomsurface of the wide trench 43 below the front surface 24 of theepitaxial layer 23 and extends into the surface layer portion of thewide electrode 49. Accordingly, a portion (lower end) of the bottomsurface and the side surface of the ground contact 17 are connected tothe wide electrode 49.

The source terminal 2 and the gate terminal 3 described above are formedon the inter-layer insulation film 50 with a gap of, e.g., about 4 μm(equal to a width W₄ of the cut-out regions 8) between the sourceterminal 2 and the gate terminal 3.

As set forth above, the source terminal 2 includes the source centerportion 15 formed directly above the active region 12 so as to cover allof the unit cells 13 arranged in a lattice pattern and the sourceextension portion 16 extending from the source center portion 15 towardthe trench isolation region 10 so as to cover a portion of the isolationtrenches 39. In the present embodiment, the source extension portion 16only covers the wide trench 43 and the narrow trench 42 adjoining thewide trench 43. Alternatively, the source extension portion 16 may coveronly the wide trench 43 or may cover the wide trench 43 and two or morenarrow trenches 42.

The source terminal 2 is simultaneously connected to the source regions31 of all of the unit cells 13 and the wide electrodes 49 through thesource contacts 53 and the ground contacts 17, respectively. That is,the source terminal 2 serves as a common terminal with respect to thesource regions 31 of all of the unit cells 13 and the wide electrodes49. This configuration allows the electric potential of the wideelectrodes 49 to be fixed at the electric potential of the sourceterminal 2. On the other hand, the source terminal 2 is not connected tothe narrow electrodes 48. Accordingly, the narrow electrodes 48surrounded by the trench insulation film 45 and the inter-layerinsulation film 50 are electrically isolated (kept floating) from otherportions of the semiconductor device 1.

The gate terminal 3 is formed directly above the marginal region 11 soas to surround the trench isolation region 10 in a spaced-apartrelationship with the trench isolation region 10.

A drain terminal 54 is formed on a rear surface 22 of the substrate 20so as to cover the entire area of the rear surface 22. The drainterminal 54 serves as a common terminal with respect to all of the unitcells 13.

<Manufacturing Method of Semiconductor Device 1>

FIGS. 3A through 3L, which show the same section view as in FIG. 2,illustrate steps of manufacturing the semiconductor device 1 shown inFIG. 1.

In the manufacture of the semiconductor device 1, as shown in FIG. 3A,silicon crystals are caused to grow on the front surface 21 of thesubstrate 20 in a wafer state by an epitaxial growth method such as aCVD (Chemical Vapor Deposition) method, an LPE (Liquid Phase Epitaxy)method or an MBE (Molecular Beam Epitaxy) method, while doping n typeimpurities on the front surface 21 of the substrate 20. Thus, the n⁻type epitaxial layer 23 (the drain region 34) is formed on the substrate20.

Next, a hard mask 55 is formed on the epitaxial layer 23 and patterned.After patterning the hard mask 55, the epitaxial layer 23 is subjectedto dry etching. Thus, the epitaxial layer 23 is selectively dug from thefront surface 24 of the epitaxial layer 23, and the gate trenches 28 andthe isolation trenches 39 are formed simultaneously. Concurrently, theplurality of unit cells 13 is formed in the epitaxial layer 23.Thereafter, the hard mask 55 is peeled off.

Next, as shown in FIG. 3B, a hard mask 56 is formed so as to cover thegate trenches 28 and the front surface 24 of the epitaxial layer 23. Ntype impurities are accelerated toward the isolation trenches 39 exposedfrom the hard mask 56, thereby implanting the n type impurities into thebottom surfaces 41 of the isolation trenches 39 (ion implantation).Subsequently, the epitaxial layer 23 is subjected to annealing. Theannealing activates the n type impurities implanted into the bottomsurfaces 41 of the isolation trenches 39, thereby forming thehigh-concentration impurity region 44 in the drain region 34.

Next, as shown in FIG. 3C, the gate insulation film 36 and the trenchinsulation film 45 are simultaneously formed on the front surface 24 ofthe epitaxial layer 23, the inner surfaces (side surfaces 29 and bottomsurfaces 30) of the gate trenches 28 and the inner surfaces (the sidesurfaces 40 and the bottom surfaces 41) of the isolation trenches 39 by,e.g., a thermal oxidation method. The thickness of the gate insulationfilm 36 and the trench insulation film 45 may range, e.g., from 150 Å to1500 Å.

Next, as shown in FIG. 3D, polysilicon (an electrode material) isdeposited on the epitaxial layer 23 by, e.g., a CVD method. Thepolysilicon is filled into the gate trenches 28 and the isolationtrenches 39, and further deposited on the front surface 24 of theepitaxial layer 23 to an appropriate thickness.

Then, the deposited polysilicon is etched back until the etched-backsurface becomes flush with the front surface 24 of the epitaxial layer23. As a consequence, the gate electrodes 37 and the buried electrodes46 made of the polysilicon remaining in the gate trenches 28 and theisolation trenches 39, respectively, and having the upper surfaces 38and 47, respectively, flush with the front surface 24 of the epitaxiallayer 23 are formed simultaneously.

Next, as shown in FIG. 3E, p type impurities are accelerated toward thefront surface 24 of the epitaxial layer 23 in a state in which no areaof the front surface 24 of the epitaxial layer 23 is covered with animplantation mask such as a hard mask or a photoresist. Thus, the p typeimpurities are implanted into the surface layer portion of the epitaxiallayer 23. Subsequently, the epitaxial layer 23 is subjected toannealing. The annealing activates the p type impurities implanted intothe surface layer portion of the epitaxial layer 23, thereby forming thechannel layer 26 in the active region 12, the trench isolation region 10and the marginal region 11. In the trench isolation region 10, thechannel layer 26 is formed between the isolation trenches 39 adjoiningeach other.

Next, as shown in FIG. 3F, a hard mask 57 is formed so as to cover anarea of the epitaxial layer 23 other than the upper surfaces of therespective unit cells 13. Then, n type impurities are accelerated towardthe upper surfaces of the unit cells 13 exposed from the hard mask 57,thereby implanting the n type impurities into the surface layer portionsof the respective unit cells 13. Subsequently, the epitaxial layer 23 issubjected to annealing. The annealing activates the p type impuritiesimplanted into the surface layer portions of the respective unit cells13, thereby forming the source regions 31 in the respective unit cells13. At this time, the upper surfaces of the dummy cells 32 are coveredwith the hard mask 57 since the source regions 31 are not formed in thedummy cells 32.

Next, as shown in FIG. 3G, a hard mask 58 is formed so as to cover anarea of the epitaxial layer 23 other than the area (the central areas ofthe upper surfaces of the respective unit cells 13) in which the channelcontact regions 35 are to be formed. Then, n type impurities areaccelerated toward the upper surfaces of the unit cells 13 exposed fromthe hard mask 58, thereby implanting the n type impurities into thesurface layer portions of the respective unit cells 13. Subsequently,the epitaxial layer 23 is subjected to annealing. The annealingactivates the p type impurities implanted into the surface layerportions of the respective unit cells 13, thereby forming the channelcontact regions 35 in the respective unit cells 13.

Next, as shown in FIG. 3H, the inter-layer insulation film 50 is formedon the epitaxial layer 23 by, e.g., a CVD method.

Next, as shown in FIG. 3I, the contact holes 51 and 52 aresimultaneously formed by selectively dry-etching the inter-layerinsulation film 50. At this time, the etching rate with respect to thegas used in etching the inter-layer insulation film 50 is larger in thepolysilicon (Poly-Si) forming of the wide electrode 49 than in thesilicon (Si) forming of the source regions 31 (Poly-Si>Si). Therefore,the contact hole 52 reaches inside to the wide electrode 49.

Subsequently. Ti/TiN barrier layers are formed on the inner surfaces ofthe respective contact holes 51 and 52. Thereafter, tungsten isdeposited so as to fill the inside of the barrier layers. Then, thedeposited tungsten is polished by a CMP (Chemical Mechanical Polishing)method until the upper surface of the tungsten becomes flush with theupper surface of the inter-layer insulation film 50. Consequently,source contacts 53 and ground contacts 17 filled into the respectivecontact holes 51 and 52 are formed simultaneously.

Next, as shown in FIG. 3J, the electrode film 4 made of AlCu (a metallicmaterial) is deposited on the entire area of the front surface of theinter-layer insulation film 50 by, e.g., a sputtering method.

Next, as shown in FIG. 3K, the cut-out region 8 is formed by patterningthe deposited electrode film 4, whereby the source terminal 2 and thegate terminal 3 are formed simultaneously.

Subsequently, as shown in FIG. 3L, the drain terminal 54 is formed onthe rear surface 22 of the substrate 20 by, e.g., a sputtering method.Thereafter, the wafer is divided into individual pieces of semiconductordevices 1 (chips) along a dicing line formed in the substrate 20 in awafer state, by using a dicing blade 59.

In this manner, an individual piece of the semiconductor device 1 shownin FIGS. 1 and 2 can be obtained, and the channel layer 26 of themarginal region 11 becomes exposed on the lateral end surfaces 5 of thesemiconductor device 1.

The MOSFET 14 of the semiconductor device 1 can be used as, e.g., aswitching element. In this case, a specified voltage (a voltage equal toor greater than a gate threshold voltage) is applied to the gateterminal 3 (the gate pad 6) in a state in which a drain voltage makingthe drain side positive between the source terminal 2 (the source pad19) and the drain terminal 54, i.e., between the source and the drain,is applied to the drain terminal 54, and the source terminal 2 is keptat a ground potential. As a consequence, a channel extending along thethickness direction of the gate trenches 28 is formed in the vicinity ofthe interface of the channel region 33 with the gate insulation film 36,and an electric current flows in the thickness direction of the gatetrenches 28.

With the semiconductor device 1, the channel layer 26 spread over fromthe active region 12 to the marginal region 11 and exposed on thelateral end surfaces 5 of the semiconductor device 1 is divided by theisolation trenches 39, whereby the active region 12 and the marginalregion 11 are electrically insulated and isolated from each other. Withthis configuration, the channel layer 26 of the active region 12 can beelectrically isolated from the channel layer 26 of the marginal region11.

While a method of electrically isolating the channel layer 26 of theactive region 12 is conventionally available, the conventional methoddiffers from the method of the present disclosure in which the isolationtrenches 39 are formed in the semiconductor device 1. The conventionalmethod suffers from a number of problems.

Specifically, in the conventional method, ion implantation for formingthe channel layer 26 is performed after a LOCOS oxide film in thesurface layer portion of the epitaxial layer 23 or a recess LOCOS oxidefilm is formed by thermal oxidation. Therefore, even if ions areaccelerated toward the entire area of the front surface 24 of theepitaxial layer 23, the accelerated ions are blocked by the LOCOS oxidefilm or the recess LOCOS oxide film. Thus the implantation of ionstoward the portions directly below the LOCOS oxide film or the recessLOCOS oxide film can be prevented. Accordingly, the channel layers 26are separately formed at the side of the active region 12 and at theside of the marginal region 11 with respect to the LOCOS oxide film. Asa result, the channel layer 26 of the active region 12 can beelectrically isolated from the channel layer 26 of the marginal region11.

In the conventional method, however, the LOCOS oxide film needs to beformed relatively thick in order to adjust the electric fields generatedfrom the source terminal 2. For this reason, it is necessary to performthe heat treatment for a long period of time when forming the LOCGOSoxide film, which may result in a deterioration of the devicecharacteristics of the semiconductor device 1.

Further, if the width of the LOCOS oxide film (the element isolationwidth) is too small, the active region 12 and the marginal region 11cannot be appropriately isolated. For this reason, a wide area isrequired for element isolation, which may result in a problem in thatthe size of the semiconductor device grows larger. In the case of, e.g.,a power device whose rated voltage exceeds 100 V, for the sake ofsecuring a high voltage resistance, it is necessary to space apart, asfar as possible, a depletion layer away from the peripheral edge of theactive region 12 in the transverse direction. In this case, even if thewidth of the LOCOS oxide film is large, the width of the LOCOS oxidefilm can be used as an enlarging space for the depletion layer. However,if the rated voltage of the power device is not high, the enlargingspace for the depletion layer needs to not be set so large. As a result,the portion of the LOCOS oxide film exceeding the width required in theenlarging space for the depletion layer becomes unnecessary.

In contrast, with the semiconductor device 1 of the present embodiment,the active region 12 and the marginal region 11 are electricallyinsulated and isolated from each other by the isolation trenches 39.Therefore, even if the width of the isolation trenches 39 is smallerthan the width of the LOCOS oxide film, the channel layers 26 can becompletely divided, and accordingly, the size of the semiconductordevice can be reduced.

Since the isolation trenches 39 are simultaneously formed with the gatetrenches 28 (see FIG. 3A), the manufacturing method of the semiconductordevice 1 can be simplified. Further, since it is not necessary toperform thermal oxidation for a long period of time, unlike the casewhere the LOCOS oxide film is formed, device characteristics (of theMOSFET 14) can be improved.

In order to assure that the channels are normally formed in the channellayers 26 (the channel region 33) by applying a gate voltage to the gateelectrodes 37, the bottom portion of the channel layers 26 may bepositioned at the same depth as the bottom surfaces 30 of the gatetrenches 28 or positioned closer to the front surface 24 of theepitaxial layer 23 than the bottom surfaces 30 of the gate trenches 28.That is, the channel layers 26 do not go around and unify at the lowerportions of the gate trenches 28.

Therefore, if the isolation trenches 39 are formed in the same step asthe step of forming the gate trenches 28 and if the depths D₁ and D₂ ofthe isolation trenches 39 and the gate trenches 28 are set equal to eachother (D₁=D₂), in the ion implantation step, the implantation conditions(such as acceleration energy and so forth) in setting the depth of thechannel layer 26 can be determined based only on the conditions requiredfor the MOSFET 14 as the channel region 33.

That is, in the ion implantation step, there is no need to set twoconditions, namely, a depth condition for allowing the channel layer 26to serve as the channel region 33 and a depth condition for allowing theisolation trenches 39 to reliably divide the channel layer 26. If theformer condition is set, the bottom portion of the channel layer 26formed under that condition does not go around the lower portions of thegate trenches 28. Further, the bottom portion of the channel layer 26does not go also around the lower portions of the isolation trenches 39having the same depth D₂ as the depth D₁ of the gate trenches 28. As aresult, the channel layer 26 in the trench isolation region 10 isreliably divided by the isolation trenches 39.

Since the source terminal 2 includes the source extension portion 16covering the trench isolation region 10, the depletion layer generatedfrom the channel layer 26 of the dummy cells 32 can be successfullywidened by the electric fields of the source extension portion 16 towardthe lateral end surfaces 5 of the semiconductor device 1 along thetransverse direction parallel to the front surface 24 of the epitaxiallayer 23. Further, inasmuch as the trench insulation film 45 is formedon the inner surfaces of the isolation trenches 39 and the buriedelectrodes 46 are filled into the inside of the trench insulation film45, the depletion layer can be successfully widened by the electricfields of the buried electrodes 46. Accordingly, the voltage resistanceof the semiconductor device 1 can be sufficiently secured.

Furthermore, by connecting the source terminal 2 to the wide electrode49 disposed outside the dummy cells 32, the electric potential of thewide electrode 49 corresponding to the inner peripheral edge of thetrench isolation region 10 can be fixed to the electric potential of thesource terminal 2 (the ground electric potential). With thisconfiguration, the electric potential of other buried electrodes 46 (thenarrow electrodes 48 in the present embodiment) disposed in the trenchisolation region 10 can be stabilized. As a result, the depletion layergenerated from the channel layer 26 of the dummy cells 32 can be stablywidened in the transverse direction.

Since the high-concentration impurity region 44 is formed in the bottomsurfaces 41 of the isolation trenches 39, the depletion layer generatedfrom the channel layer 26 of the dummy cells 32 can be widened in thetransverse direction so as to avoid the high-concentration impurityregion 44. Accordingly, electric field concentration on the bottomsurfaces 41 of the isolation trenches 39 can be reduced.

In the semiconductor device 1, the high-concentration impurity region 44may be omitted, as shown in FIG. 4 or a high-concentration impurityregion 60 (a second high-concentration impurity region) may be formedbelow the gate trenches 28, as shown in FIG. 5A. In the example shown inFIG. 5A, it is not necessary to cover the bottom surfaces 30 of the gatetrenches 28 with a hard mask when forming the high-concentrationimpurity regions 44 and 60. The n type impurity may be implanted usingthe hard mask 55 formed in the step shown in FIG. 3A (the hard mask 55used in forming the gate trenches 28 and the isolation trenches 39).Accordingly, the number of steps in manufacturing the semiconductordevice 1 can be reduced.

As shown in FIG. 5B, the trench insulation film 45 may include arelatively thick bottom portion 451 formed on the bottom surfaces 41 ofthe isolation trenches 39 and a relatively thin side portion 452 thinnerthan the bottom portion 451. The side portion 452 is formed on the sidesurfaces 40 of the isolation trenches 39. In other words, the thicknesst₁ of the bottom portion 451 is larger than the thickness t₂ of the sideportion 452. This is because, due to the formation of thehigh-concentration impurity region 44 on the bottom surfaces 41 of theisolation trenches 39, the oxidation rate of the bottom surfaces 41 ishigher than the oxidation rate of the side surfaces 40 when the innersurfaces (the side surfaces 40 and the bottom surfaces 41) of theisolation trenches 39 are subjected to thermal oxidation (see FIG. 3C).Since the bottom portion 451 can be formed to be relatively thick, thevoltage resistance of the trench insulation film 45 can increase. Thedifference in the thickness of the bottom portion 451 and the sideportion 452 may be available in all of the isolation trenches 39. Inparticular, the thickness difference may be employed directly below thewide electrode 49 in order to apply a voltage having the same magnitudeas the magnitude of the voltage applied between the source and the drainto the wide electrode 49 connected to the source terminal 2 through theground contacts 17. The thickness difference may be employed in theconfigurations shown in FIGS. 2 and 12.

In FIG. 1, like the cut-out regions 8, the trench isolation region 10 isset such that one corner portion of the rectangular trench isolationregion 10, including the isolation trenches 39, extending along thelateral end surfaces 5 of the semiconductor device 1 is recessed towardthe source terminal 2 so as to inwardly bypass the gate pad 6 (bypassthe gate pad 6 at the side of the active region 12 with respect to thegate pad 6). In an alternative example, as shown in FIG. 6, the trenchisolation region 10, including the isolation trenches 39, may be setsuch that the rectangular trench isolation region 10 extends along thefour lateral end surfaces 5 of the semiconductor device 1 at a sideopposite to the active region 12 with respect to the gate pad 6 so as togo around the gate pad 6 at the outer side of the gate pad 6 (at theside of the lateral end surfaces 5 with respect to the gate pad 6) sothat the trench isolation region 10 outwardly bypasses the gate pad 6.

<Other Embodiments of Semiconductor Device 1>

The semiconductor device 1 of the present disclosure may also beembodied in the forms shown in FIGS. 7 through 12. In FIGS. 7 through12, the portions corresponding to the respective portions shown in FIGS.1 through 6 will be designated by the same reference numerals andsymbols. Detailed description on the portions designated by the samereference numerals and symbols will be omitted below.

(1) Embodiment without Wide Trench 43 (FIGS. 7 and 8)

In a semiconductor device 61 shown in FIG. 7, for example, the width W₂of the isolation trenches 39 may be uniform and the trench pitch P₂ maybe uniform.

In this case, the width W₂ of the isolation trenches 39 may be equal tothe width W₁ of the gate trenches 28, and the trench pitch P₂ may besmaller than the pitch P₁ of the gate trenches 28.

The buried electrodes 46 may be formed only directly below the cut-outregion 8 and may be electrically isolated (kept floating) from otherportions of the semiconductor device 61. That is, the source terminal 2may not include the source extension portion 16. Alternatively, as in asemiconductor device 62 shown in FIG. 8, the gate terminal 3 may includea gate center portion 63 formed directly above the marginal region 11and a gate extension portion 64 extending from the gate center portion63 toward the trench isolation region 10 so as to cover a portion of theisolation trenches 39.

(2) Embodiment with Decreasing Trench Pitch (FIG. 9)

In a semiconductor device 65 shown in FIG. 9, for example, the pitch ofthe isolation trenches 39 may decrease step by step from the activeregion 12 toward the marginal region 11 in such a way as to comply withthe inequality P₂>P₂′>P₂″>P₂′″ . . . .

Since the pitch of the isolation trenches 39 in the semiconductor device65 decreases step by step as described above, even in a positionrelatively distant from the dummy cells 32, the depletion layergenerated from the dummy cells 32 can be stably widened.

(3) Embodiment with Increasing Trench Pitch (FIG. 10)

In a semiconductor device 66 shown in FIG. 10, for example, the pitch ofthe isolation trenches 39 may increase step by step from the activeregion 12 toward the marginal region 11 in such a way as to comply withthe inequality P₂<P₂′<P₂″<P₂′″ . . . .

(4) Embodiment with Single Trench (FIG. 11)

In a semiconductor device 67 shown in FIG. 11, for example, only asingle isolation trench 39 may be provided. Insofar as the singleisolation trench 39 can divide the channel layer 26 between the activeregion 12 and the marginal region 11, the single isolation trench 39 maybe formed directly below the cut-out region 8, as shown in FIG. 11, ormay be formed directly below the source terminal 2 or the gate terminal3.

(5) Embodiment with IGBT (Insulated Gate Bipolar Transistor) (FIG. 12)

As shown in FIG. 12, for example, a trench gate type IGBT 68, instead ofthe trench gate type MOSFET 14, may be formed in the active region 12 ofa semiconductor device 69.

The semiconductor device 69 shown in FIG. 12 includes a substrate 70made of p⁺ type silicon in place of the n⁺ type substrate 20. Theemitter regions 71 correspond to source regions 31 of the MOSFET 14.Base regions 72 correspond to the channel regions 33. A collector region73 corresponds to the drain region 34. An emitter terminal 74corresponds to the source terminal 2. A collector terminal 75corresponds to the drain terminal 54.

In the semiconductor device 69, the channel layer 26 exposed on thelateral end surfaces 5 of the semiconductor device 69 is divided by theisolation trenches 39. Accordingly, the same advantageous effects asthose available in the semiconductor device 1 of the previous embodimentcan be obtained.

<Relationship Between Trench Number, Pitch and Breakdown Voltage>

FIG. 13 illustrates a graph showing the changes of a breakdown voltage(BVDSS) when the number and the pitch of the isolation trenches 39 ischanged.

As shown in FIG. 13, the changes of the breakdown voltage depending onthe changes of the number of the isolation trenches 39 were inspectedusing the semiconductor devices 1 with completely identical structures(the structure shown in FIG. 2) except the following conditions (1)through (4). An electric current of 1 nA was caused to flow between thesource and the drain. Conditions (1) through (4) are as follows:

Condition (1): the buried electrodes 46 were kept electrically floatingand the pitch of the isolation trenches 39 was set equal to 0.9 μm(equal to the pitch of the gate trenches 28);

Condition (2): only the innermost buried electrode 46 was connected tothe source terminal 2 to be kept at the ground electric potential (theother buried electrodes 46 were kept floating) and the pitch of theisolation trenches 39 was set equal to 0.9 μm (equal to the pitch of thegate trenches 28);

Condition (3): only the innermost buried electrode 46 was connected tothe source terminal 2 to be kept at the ground electric potential (theother buried electrodes 46 were kept floating) and the pitch of theisolation trenches 39 was set equal to 0.7 μm (smaller than the pitch ofthe gate trenches 28); and

Condition (4): only the innermost buried electrode 46 was connected tothe source terminal 2 to be kept at the ground electric potential (theother buried electrodes 46 were kept floating) and the pitch of theisolation trenches 39 was set equal to 0.5 μm (smaller than the pitch ofthe gate trenches 28).

Referring to FIG. 13, it can be noted that, if the pitch of theisolation trenches 39 is small (equal to or less than 0.7 μm), thevoltage resistance of the semiconductor device 1 can be made larger thanthe voltage resistance available in the case where the pitch is greaterthan 0.7 μm. While certain embodiments of the present disclosure havebeen described above, the present disclosure can be embodied in otherforms. The pitch of the isolation trenches 39 is not limited to theexamples shown in FIGS. 2 and 7 through 10. For example, a first pitch(a relatively wide pitch) and a second pitch smaller than the firstpitch (a relatively narrow pitch) may be arranged alternately.

In the case where the source terminal 2 is connected to the buriedelectrode 46, the buried electrode 46 connected to the source terminal 2does not need to be employed in the innermost trench. The buriedelectrode 46 connected to the source terminal 2 may be employed in theoutermost trench (the trench disposed nearest to the marginal region 11)or may be employed in an intermediate trench disposed between theinnermost trench and the outermost trench.

Configurations in which the conductivity types of the respectivesemiconductor portions of the semiconductor devices 1, 61, 62, 65, 66,67 and 69 are inverted may also be employed. For example, in thesemiconductor device 1, the p type portion may be an n type and the ntype portion may be a p type.

In the semiconductor devices 1, 61, 62, 65, 66, 67 and 69, the layermaking up the epitaxial layer 23 is not limited to the epitaxial layer23 made of Si but may be a layer made of, e.g., a wide-band-gapsemiconductor such as SiC, GaN or diamond.

The features grasped from the foregoing embodiments can be used incombination in the respective embodiments. The components appearing inthe respective embodiments can also be combined within the scope of thepresent disclosure.

In addition, many different changes in design can be made withoutdeparting from the scope of the subject matters defined in the claims.

Disclosure According to Reference Example Background Art of ReferenceExample

There are conventionally known semiconductor devices having agate-protecting zener diode.

For example, there is known a power MOSFET that includes an n⁺ typesemiconductor substrate, a drain region formed of n⁻ type epitaxiallayers and a p type channel layer laminated on the semiconductorsubstrate one above another, a gate trench extending from the channellayer to the drain region, a gate electrode filled into the gate trenchand made of polysilicon, an n⁺ type source region formed on the frontsurface of the channel layer, a source electrode making contact with thesource region, a diode trench extending from the channel layer to thedrain region, and a polysilicon layer attached to the inner wall of thediode trench.

The polysilicon layer is formed such that one end and the other endthereof protrude from the inner wall of the diode trench toward thefront surface of the epitaxial layer. The one end and the other end ofthe polysilicon layer protruding toward the front surface of theepitaxial layer are connected to a source electrode and a gateelectrode, respectively. The polysilicon layer includes an n⁺ typeregion, a p⁻ type region, an n⁺ type region, a p⁻ type region, an n⁺type region and a p⁻ type region arranged from the one end to the otherend in a zebra pattern. Thus, six zener diodes (pn junctions) areserially connected in the polysilicon layer.

Embodiments of Reference Example

Certain embodiments of the reference example will now be described indetail with reference to the accompanying drawings.

<Overall Plan-View Configuration of Semiconductor Device>

FIG. 14 illustrates a schematic plan view of a semiconductor deviceaccording to an embodiment of the reference example.

The semiconductor device 101 has, e.g., a rectangular chip shape whenseen in a plan view. The widths of the chip-shaped semiconductor device101 in the up-down and the left-right directions on the sheet surface inFIG. 14 are approximately several millimeters, respectively.

On the front surface of the semiconductor device 101, there is formed anelectrode film 104 including a source terminal 102 as one example of asecond terminal and a gate terminal 103 as one example of a firstring-shaped terminal. The electrode film 104 is formed such that thesource terminal 102 is surrounded by the rectangular ring-shaped gateterminal 103 extending along the peripheral edge of the semiconductordevice 101. A gate pad 105 is installed in one corner portion of thegate terminal 103. Bonding wires 106 are connected to the gate pad 105.In FIG. 14, the source terminal 102 is indicated transparently to showthe unit cells 112 of a MOSFET 113, to be described later.

Cut-out regions 107 are formed between the source terminal 102 and thegate terminal 103 and at the outer side of the gate terminal 103 (seethe cross-hatched portions in FIG. 14). The cut-out regions 107 refer tothe portions from which the electrode film 104 is removed to prevent thesource terminal 102 and the gate terminal 103 from making contact witheach other.

In the present embodiment, the cut-out region 107 disposed between thesource terminal 102 and the gate terminal 103 is formed into such ashape that one corner portion of the rectangular cut-out region 107extending along the peripheral edge of the semiconductor device 101 isrecessed toward the source terminal 102 so as to bypass the gate pad105. The gate pad 105 is installed in a pad space 108 of the gateterminal 103 formed by recessing the cut-out region 107, as describedabove.

A ring-shaped diode region 109 extending along the cut-out regions 107is defined in the semiconductor device 101. An active region 110 isdefined in the portion surrounded by the diode region 109.

A ring-shaped double-ended zener diode 111 is formed in the diode region109 over the entire circumference of the diode region 109. Like thecut-out region 107, the diode region 109 is formed so as to bypass thegate pad 105. Accordingly, a force transferred to the diode region 109(the double-ended zener diode 111) when an impact is applied to the gatepad 105, e.g., in the course of bonding the wires 106 to the gate pad105, can be reduced.

A MOSFET 113, in which the plurality of unit cells 112 is arranged in alattice pattern, is formed in the active region 110. The arrangementpattern of the unit cells 112 is not limited to a lattice pattern butmay be, e.g., a stripe pattern or a zigzag pattern.

The cut-out region 107 disposed between the source terminal 102 and thegate terminal 103 is formed into a zigzag shape over substantially theentire circumference of the cut-out region 107. Thus, the sourceterminal 102 and the gate terminal 103 partitioned by the cut-out region107 include a plurality of source extension portions 114 extendinglinearly from the peripheral edge portion of the source terminal 102toward the gate terminal 103 and a plurality of gate extension portions115 extending linearly from the peripheral edge portion of the gateterminal 103 toward the source terminal 102, respectively. The sourceextension portions 114 and the gate extension portions 115 arealternately arranged along the circumferential direction of the gateterminal 103 so as to mesh with each other in a comb-like manner. Thesource extension portions 114 and the gate extension portions 115 areformed to extend across the border of the active region 110 and thediode region 109. The source extension portions 114 are one example ofsecond extension portions and the gate extension portions 115 are oneexample of first extension portions.

The source extension portions 114 include source contacts 116 arrangeddirectly above the diode region 109. The source contacts 116 areconnected to the double-ended zener diode 111. In each of the sourceextension portions 114, the source contacts 116 are formed in pluralnumbers (two, in the present embodiment) and arranged along thelongitudinal direction running from the tip end of each of the sourceextension portions 114 toward the base end thereof. The number of thesource contacts 116 arranged in each of the source extension portions114 corresponds to the number of diode trenches 135, to be describedlater.

Source pads 118, to which bonding wires 117 are connected, are installedin the portion of the source terminal 102 directly above the MOSFET 113.The source pads 118 are provided in plural numbers (three, in thepresent embodiment) and arranged side by side, e.g., from a positiondiagonal to the pad space 108 along the peripheral edge of the sourceterminal 102.

The gate extension portions 115 includes MOS gate contacts 119 arrangeddirectly above the active region 110 and gate contacts 120 arrangeddirectly above the diode region 109. The MOS gate contacts 119 areconnected to a gate electrode 133 (to be described later) of the MOSFET113 by gate extension wiring lines 121 extending across the cut-outregion 107. The gate contacts 120 are connected to the double-endedzener diode 111. In each of the gate extension portions 115, the MOSgate contacts 119 and the gate contacts 120 are formed in that orderalong the longitudinal direction running from the tip end (near the unitcells 112) of each of the gate extension portions 115 toward the baseend thereof. In each of the gate extension portions 115, the gatecontacts 120 are formed in plural numbers (two, in the presentembodiment) and arranged along the longitudinal direction. Like thesource contacts 116, the number of the gate contacts 120 of each of thegate extension portions 115 corresponds to the number of the diodetrenches 135, to be described later.

<Partial Plan-View and Section-View Configuration of SemiconductorDevice>

FIG. 15 illustrates a partially enlarged view of the semiconductordevice shown in FIG. 14. FIG. 16A illustrates a schematic section viewof the semiconductor device taken along line A-A in FIG. 15. FIG. 16Billustrates a schematic section view of the semiconductor device takenalong line B-B in FIG. 15. FIG. 17 illustrates a circuit diagram forexplaining an equivalent circuit of the semiconductor device shown inFIG. 14.

In the semiconductor device 101, the MOSFET 113 is formed in the activeregion 110. The double-ended zener diode 111 is formed in the dioderegion 109. As elements shared by the MOSFET 113 and the double-endedzener diode 111, the semiconductor device 101 includes a substrate 122made of n⁺ type silicon (e.g., having a concentration ranging from1.0×10¹⁹ to 1.0×10²¹ atoms·cm⁻³, which shall apply hereinafter), and anepitaxial layer 124 made of n⁻ type silicon (e.g., having aconcentration ranging from 5.0×10¹⁶ to 5.0×10¹⁴ atoms·cm⁻³, which shallapply hereinafter) and formed on a front surface (upper surface) 123 ofthe substrate 122. The thickness of the epitaxial layer 124 as asemiconductor layer may range, e.g., from 3 μm to 100 μm. N typeimpurities, e.g., phosphor (P) or arsenic (As), are included in thesubstrate 122 and the epitaxial layer 124.

The MOSFET 113 formed in the active layer 110 is a trench gate typeMOSFET (Metal Oxide Semiconductor Field Effect Transistor). In theepitaxial layer 124, gate trenches 126 dug from a front surface 125 ofthe epitaxial layer 124 toward the substrate 122 are formed in a latticepattern. The gate trenches 126 may be formed in a stripe pattern. In theepitaxial layer 124, a plurality of the unit cells 112, each unit cell112 having a rectangular parallelepiped shape (a square shape when seenin a plan view), is formed in the respective window portions surroundedby the lattice-like gate trenches 126.

The depth D₁ of the gate trenches 126 measured from the front surface125 of the epitaxial layer 124 may range, e.g., from 0.5 μm to 5.0 μm.The width W₁ of the gate trenches 126 may range, e.g., from 0.10 μm to1.0 μm. The distance between the unit cells 112 adjoining each other(the cell pitch P₁) may range, e.g., from 0.5 μm to 5.0 μm.

In the epitaxial layer 124 around the gate trenches 126, an n⁺ typesource region 127 and a p type channel region 128 (e.g., having aconcentration ranging from 1.0×10¹⁵ to 1.0×10¹⁸ atoms·cm⁻³, which shallapply hereinafter) are formed in that order from the side near the frontsurface 125 of the epitaxial layer 124. P type impurities such as boron(B) or aluminum (Al) are included in the channel region 128.

The source region 127 is formed in the surface layer portion of each ofthe unit cells 112 so as to be exposed on the front surface 125 of theepitaxial layer 124 and form the upper portion of the side surface ofeach of the gate trenches 126. The thickness of the source region 127 inthe direction running from the front surface 125 toward the substrate122 may range, e.g., from 0.1 μm to 1.0 μm. The term “thickness” definedin the following description refers to a thickness in the directionrunning from the front surface 125 of the epitaxial layer 124 toward thesubstrate 122, unless specified otherwise.

The channel region 128 is formed at the side of the substrate 122 (atthe side of a rear surface 129 of the epitaxial layer 124) with respectto the source region 127 so as to make contact with the source region127. The thickness of the channel region 128 may range, e.g., from 0.2μm to 1.0 μm. The bottom portion of the channel region 128 is positionedcloser to the front surface 125 of the epitaxial layer 124 than thebottom surface of each of the gate trenches 126 is.

The region of the epitaxial layer 124 disposed at the side of thesubstrate 122 with respect to the channel region 128 is an n⁻ type driftregion 130 in which the state available after the epitaxial growth iskept as is. The drift region 130 is arranged at the side of thesubstrate 122 with respect to the channel region 128 and makes contactwith the channel region 128.

A p⁺ type body contact region 131 (e.g., having a concentration rangingfrom 1.0×10¹⁸ to 1.0×10²⁰ atoms·cm⁻³, which shall apply hereinafter)extending from the front surface 125 of the epitaxial layer 124 to thechannel region 128 through the source region 127 is formed in thecentral portion of each of the unit cells 112.

A gate insulation film 132 made of silicon oxide (SiO₂) is formed on theinner surface of each of the gate trenches 126 so as to cover the entirearea of the inner surface of each of the gate trenches 126. The insideof the gate insulation film 132 is filled with polysilicon doped withn-type impurities at a high concentration, whereby a gate electrode 133is buried in each of the gate trenches 126.

The gate electrode 133 has an upper surface 134 flush with the frontsurface 125 of the epitaxial layer 124. The term “flush” in thisdescription means, for example, that the upper surface 134 of the gateelectrode 133 and the front surface 125 of the epitaxial layer 124 areformed on the same plane. However, the term “flush” conceptually mayalso include the case where a small depression is unintentionally formedin the course of forming the gate electrode 133. A portion of the gateelectrode 133 protrudes as the gate extension wiring line 121 from theframe portion of a lattice to directly below the gate extension portion115 so as to extend across the cut-out region 107 when seen in a planview.

A plurality (two, in the present embodiment) of diode trenches 135 dugfrom the front surface 125 of the epitaxial layer 124 toward thesubstrate 122 is formed in the epitaxial layer 124 of the diode region109. The diode trenches 135 are provided in ring-shaped patterns so asto surround the MOSFET 113 (the active region 110) and are arranged toextend over the entire circumference of the diode region 109 with auniform gap left therebetween. The distance (trench pitch P₂) betweenthe centers of the diode trenches 135 adjoining each other may range,e.g., from 0.5 μm to 5.0 μm. Each of the diode trenches 135 is formed insuch a position as to extend across the source extension portions 114and the gate extension portions 115 when seen in a plan view. The numberof the diode trenches 135 is not limited to two but may be one or threeor more. Accordingly, the number of the double-ended zener diodes 111,to be described later, is not limited to two but may be one or three ormore.

The depth D₂ of the diode trenches 135 measured from the front surface125 of the epitaxial layer 124 is equal to the depth D₁ of the gatetrenches 126 (e.g., from 0.5 μm to 5.0 μm). The width W₂ of the diodetrenches 135 may range, e.g., from 1.2 to 5.0 times as large as thewidth W₁ of the gate trenches 126. In some embodiments, the width W₂ ofthe diode trenches 135 may range from 0.12 pn to 5.0 μm.

A diode insulation film 136 made of silicon oxide (SiO₂) is formed onthe inner surface of each of the diode trenches 135 so as to cover theentire area of the inner surface.

The inside of the diode insulation film 136 is filled with polysiliconas a buried layer doped with n type impurities and p type impurities,whereby the double-ended zener diodes 111 made of polysilicon are formedin the diode trenches 135. Like the gate electrode 133, each of thedouble-ended zener diodes 11 has an upper surface 137 flush with thefront surface 125 of the epitaxial layer 124.

Each of the double-ended zener diodes 111 as first diodes includes aplurality of n⁺ type portions 138 and a plurality of p⁻ type portions139 (e.g., having a concentration ranging from 1.0×10¹⁶ to 1.0×10¹⁹atoms·cm⁻³, which shall apply hereinafter), which are alternatelyarranged along the circumferential direction of each of the diodetrenches 135 so as to make contact with one another. In the double-endedzener diodes 111, therefore, zener diodes formed of pn junctions of then⁺ type portions 138 and the p⁻ type portions 139 are serially connectedto one another along the circumferential direction of each of the diodetrenches 135.

In the present embodiment, a diode unit 140 (a single unit) is formedof, e.g., four junctions including an np junction, a pn junction, an npjunction and a pn junction, which are made up of five continuouspartitioned portions including an n⁺ type portion 138, a p⁻ type portion139, an n⁺ type portion 138, a p⁻ type portion 139 and an n⁺ typeportion 138. That is, the double-ended zener diodes 1 are formed of aplurality of diode units 140 arranged along the circumferentialdirection of each of the diode trenches 135. The n⁺ type portions 138disposed at the ends of the respective diode units 140 are shared by thediode units 140 adjoining each other.

In order for the diode units 140 to have a specified voltage resistance,the length L₁ of each of the diode units 140 in the circumferentialdirection of each of the diode trenches 135 may range, e.g., from 10 μmto 50 μm. The lengths l₁ and l₂ of the n⁺ type portions 138 and the p⁻type portions 139, respectively, in the circumferential direction ofeach of the diode trenches 135 may range, e.g., from 3 μm to 5 μm,respectively.

The zener breakdown voltage of the diode units 140 may range, e.g., from5 V to 50 V. The zener breakdown voltage per pn junction of each of thediode units 140 may range, e.g., from 5 V to 10 V.

The combination of the n type portions and the p type portions making upeach of the diode units 140 is not limited to the combination of the n⁺type portions 138 and the p⁻ type portions 139. The n⁺ type portions 138and the p⁻ type portions 139 can be replaced by an n type portion (e.g.,having a concentration ranging from 1.0×10¹⁶ to 1.0×10¹⁹ atoms·cm⁻³,which shall apply hereinafter), an n⁻ type portion, a p type portion anda p⁺ type portion. For example, p⁺ type portions may be arranged at theopposite ends of each of the diode units. Each of the diode units may beformed of four junctions including a pn junction, an np junction, a pnjunction and an np junction, which are made up of five continuouspartitioned portions including a p⁺ type portion, an n⁻ type portion, ap⁺ type portion, an n⁻ type portion and a p⁺ type portion.

An inter-layer insulation film 141 made of SiO₂ is formed on theepitaxial layer 124.

In the inter-layer insulation film 141, contact holes 142 through 145extending through the inter-layer insulation film 141 in the thicknessdirection thereof are formed directly above the source region 127 ofeach of the unit cells 112, the n⁺ type portions 138 of the oppositeends of each of the diode units 140 and the gate protrusion wiring line121.

The source terminal 102 and the gate terminal 103 described above areformed on the inter-layer insulation film 141.

The source terminals 102 are collectively connected to the sourceregions 127 of all of the unit cells 112 through the contact holes 142and are connected as source contacts 116 to the n⁺ type portions 138disposed at one end of each of the diode units 140 through the contactholes 143. That is, the source terminals 102 are common terminals withrespect to the source regions 127 of all of the unit cells 112 and thediode units 140. In the following description, the n⁺ type portion 138of each of the diode units 140 connected to the source terminal 102 willbe also referred to as a source-side n⁺ type portion 138S.

The gate terminals 103 are connected as MOS gate contacts 119 to thegate extension wiring lines 121 through the contact holes 144 and areconnected as gate contacts 120 to the n⁺ type portions 138 disposed atthe opposite end of each of the diode units 140 from the source-side n⁺type portion 138S through the contact holes 145. That is, the gateterminals 103 are common terminals with respect to the gate electrodes133 and the diode units 140. In the following description, the n⁺ typeportion 138 of each of the diode units 140 connected to the gateterminal 103 will be also referred to as a gate-side n⁺ type portion138G.

A drain terminal 147 is formed on a rear surface 146 of the substrate122 so as to cover the entire area of the rear surface 146 of thesubstrate 122. The drain terminal 147 is a common terminal with respectto all of the unit cells 112.

In the semiconductor device 101 described above, as shown in FIG. 17,the double-ended zener diode 111 (Z_(D)) is connected between the sourceterminal 102 (S) and the gate terminal 103 (G) (between the source andthe gate). Reference symbol D₁ designates a substrate diode which isconnected between the source terminal 102 (S) and the drain terminal 147(D).

<Manufacturing Method of Semiconductor Device>

FIGS. 18A and 18B through 26A and 26B, which show the same section viewas in FIGS. 16A and 16B, illustrate steps of manufacturing thesemiconductor device shown in FIG. 14.

In the manufacture of the semiconductor device 101, as shown in FIGS.18A and 18B, silicon crystals are caused to grow on the front surface123 of the substrate 122 by an epitaxial growth method such as a CVD(Chemical Vapor Deposition) method, an LPE (Liquid Phase Epitaxy) methodor an MBE (Molecular Beam Epitaxy) method, while doping n typeimpurities on the front surface 123 of the substrate 122. Thus the n⁻type epitaxial layer 124 (the drift region 130) is formed on thesubstrate 122.

Next, as shown in FIGS. 19A and 19B, p type impurities are implantedtoward the front surface 125 of the epitaxial layer 124. The p typeimpurities are one-step implanted from the front surface 125 to aspecified depth position at an energy level ranging, e.g., from 30 keVto 200 keV. At this time, the dosage may range, e.g., from 1.0×10¹²atoms·cm⁻² to 1.0×10¹⁴ atoms·cm⁻². Subsequently, the epitaxial layer 124is annealed at a temperature ranging, e.g., from 850 degrees C to 1100degrees C. This activates the p type impurities implanted into thesurface layer portion of the epitaxial layer 124, thereby forming thechannel region 128.

Next, as shown in FIGS. 20A and 20B, a hard mask (not shown) is formedon the epitaxial layer 124. After patterning the hard mask, theepitaxial layer 124 is subjected to dry etching. Thus the epitaxiallayer 124 is selectively dug from the front surface 125 of the epitaxiallayer 124, and the gate trenches 126 and the diode trenches 135 areformed simultaneously. At the same time, the plurality of unit cells 112is formed in the epitaxial layer 124.

Next, as shown in FIGS. 21A and 21B, the gate insulation film 132 andthe diode insulation film 136 are simultaneously formed by, e.g., athermal oxidation method, on the front surface 125 of the epitaxiallayer 124, the inner surfaces (the side surfaces and the bottomsurfaces) of the gate trenches 126 and the inner surfaces (the sidesurfaces and the bottom surfaces) of the diode trenches 135. Thethickness of the gate insulation film 132 and the diode insulation film136 may range, e.g., from 150 Å to 1500 Å.

Next, as shown in FIGS. 22A and 22B, p⁻ type polysilicon 148 (asemiconductor material) is deposited on the epitaxial layer 124 by,e.g., a CVD method. The polysilicon 148 is filled into the gate trenches126 and the diode trenches 135 and is deposited until the thickness T₁from the front surface 125 of the epitaxial layer 124 becomes equal toabout 6000 Å.

Next, as shown in FIGS. 23A and 23B, the deposited polysilicon 148 isetched back until the etch-back surface becomes flush with the frontsurface 125 of the epitaxial layer 124. Thus, the gate electrode 133 anda buried layer 149 made of the polysilicon 148 remaining in the gatetrenches 126 and the diode trenches 135, respectively, are formedsimultaneously. The gate electrode 133 and the buried layer 149 arerespectively provided with upper surfaces 134 and 137 flush with thefront surface 125 of the epitaxial layer 124.

Next, as shown in FIGS. 24A and 24B, a hard mask 150 is formed on theepitaxial layer 124. After patterning the hard mask 150, n typeimpurities are implanted into the gate electrode 133 exposed from thehard mask 150. At the same time, n type impurities having aconcentration higher than the impurity concentration of the buried layer149 are selectively implanted into the buried layer 149. The n typeimpurities are one-step implanted from the front surface 125 to aspecified depth position at an energy level ranging, e.g., from 30 keVto 200 keV. At this time, the dosage may range, e.g., from 5.0×10¹⁴atoms·cm⁻² to 5.0×10¹⁶ atoms·cm⁻². Thus the n⁺ type gate electrode 133is formed. At the same time, the double-ended zener diode 111, in whichthe n⁺ type portions 138 and the p⁻ type portions 139 are arrangedalternately, is formed in the buried layer 149.

Next, as shown in FIGS. 25A and 25B, n type impurities and p typeimpurities are sequentially and selectively implanted toward the frontsurface 125 of the epitaxial layer 124. Subsequently, the epitaxiallayer 124 is annealed at a temperature ranging, e.g., from 850 degrees Cto 1100 degrees C. This activates the n type impurities and the p typeimpurities thus implanted, thereby forming the source region 127 and thebody contact region 131.

Thereafter, the inter-layer insulation film 141 is deposited on theepitaxial layer 124 by, e.g., a CVD method. Then, the inter-layerinsulation film 141 is selectively dry-etched to form the contact holes142 through 145.

Next, as shown in FIGS. 26A and 26B, the electrode film 104 made of AlCu(a metallic material) is deposited on the entire area of the frontsurface of the inter-layer insulation film 141 by, e.g., a sputteringmethod. The electrode film 104 is deposited until at least all of thecontact holes 142 through 145 are filled with the electrode film 104 andthe thickness of the electrode film 104 from the front surface 125 ofthe epitaxial layer 124 becomes equal to about 30000 Å. The cut-outregions 107 are formed by patterning the deposited electrode film 104,whereby the source terminal 102 and the gate terminal 103 are formedsimultaneously. Thereafter, the drain terminal 147 is formed on the rearsurface 146 of the substrate 122 by, e.g., a sputtering method.

Through the steps described above, the semiconductor device 101 shown inFIG. 14 can be obtained.

The MOSFET 113 of the semiconductor device 101 can be used as, e.g., aswitching element. In this case, a specified voltage (a voltage equal toor greater than a gate threshold voltage) is applied to the gateterminal 103 (the gate pad 105) in a state in which a drain voltagemaking the drain side positive between the source terminal 102 (thesource pad 118) and the drain terminal 147, i.e., between the source andthe drain, is applied to the drain terminal 147. As a consequence, achannel extending along the thickness direction of the gate trenches 126is formed in the vicinity of the interface of the channel region 128with the gate insulation film 132, and an electric current flows in thethickness direction of the gate trenches 126.

With the semiconductor device 101, as shown in FIG. 17, the double-endedzener diode 111 (Z_(D)) is connected between the source terminal 102 (S)and the gate terminal 103 (G) (between the source and the gate). Forthis reason, even if static electricity or a surge voltage is input intothe semiconductor device 101, the double-ended zener diode 111 canabsorb the static electricity or the surge voltage. Accordingly, adielectric breakdown, which would otherwise be caused by the staticelectricity or the surge voltage input to the semiconductor device 101,can be prevented. As a result, the reliability of the semiconductordevice 101 can be enhanced.

With the semiconductor device 101, the upper surface 134 of the gateelectrode 133 and the upper surface 137 of the double-ended zener diode11 are flush with the front surface 125 of the epitaxial layer 124.Accordingly, the gate electrode 133 and the double-ended zener diode 111(the buried layer 149 making up the body of the double-ended zener diode111) can be formed simultaneously (the step shown in FIG. 18F) byetching the epitaxial layer 124 from the front surface 125 of theepitaxial layer 124 to simultaneously form the gate trenches 126 and thediode trenches 135 (the step shown in FIGS. 20A and 20B), depositing thep⁻ type polysilicon 148 so as to fill the trenches 126 and 135 (the stepshown in FIGS. 22A and 22B), and then etching back the depositedpolysilicon 148.

Therefore, the manufacturing process of the semiconductor device can besimplified and the manufacturing cost can be reduced as compared withthe conventional method, in which a step of forming a gate electrode sothat the upper surface thereof becomes flush with the front surface ofan epitaxial layer and a step of forming a zener diode so as to protrudefrom a diode trench toward the front surface of the epitaxial layer areperformed independently of each other.

In the conventional method, the zener diode is shaped to protrude fromthe diode trench. For this reason, after depositing polysilicon so as tofill the diode trench, it is necessary to perform a step of patterningthe polysilicon using a mask, which deteriorates efficiency.

Further, in the conventional method, the zener diode protruding from thediode trench is left as a step difference with respect to the frontsurface of the epitaxial layer. As a result, the step difference (aheight deviation) is generated in the inter-layer insulation film formedso as to cover the front surface of the epitaxial layer. For thisreason, it is likely that a focus abnormality may occur when aphotoresist is exposed to form contact holes in the inter-layerinsulation film. In addition, it is likely that the inter-layerinsulation film will have a discontinuous portion at the step differenceand a wiring metal will easily come into the discontinuous portion.Therefore, it is likely that the wiring metal is left as a residue(metal residue) when the source electrode or the like is formed bypatterning the wiring metal, and thus the wiring lines mayshort-circuit.

In contrast, with the present manufacturing method described above, theupper surface 134 of the gate electrode 133 and the upper surface 137 ofthe double-ended zener diode 111 can be made flush with the frontsurface 125 of the epitaxial layer 124. Accordingly, the focusabnormality problem and the metal residue problem inherent in theconventional method can be solved. As a result, the quality andperformance of the semiconductor device 101 can be improved.

In the present manufacturing method described above, the gate trenches126 and the diode trenches 135 are formed simultaneously (the step shownin FIGS. 20A and 20B), and the source terminal 102 and the gate terminal103 are also formed simultaneously (the step shown in FIGS. 26A and26B). Accordingly, the semiconductor device 101 can be more easily andcost-effectively manufactured.

Further, since the width W₂ of the diode trenches 135 is 1.2 to 5.0times as large as the width W₁ of the gate trenches 126, the variationsof the width W₂ of the diode trenches 135 and the width W₁ of the gatetrenches 126 can be kept within a specified range. Accordingly, theoccurrence of dishing in the gate electrode 133 and the double-endedzener diode 111 (the imbedding layer 149) when etching back thepolysilicon 148 (the step shown in FIGS. 23A and 23B) can be prevented.

With the semiconductor device 101, the source-side n⁺ type portion 138Sof each of the diode units 140 and the source terminal 102 can beconnected to each other, and the gate-side n⁺ type portion 138G and thegate terminal 103 can also be connected to each other using the columnarplug contacts 116 and 120 extending directly downward from the sourceextension portions 114 and the gate extension portions 115 through theinter-layer insulation film 141. Therefore, it is not necessary to drawthe wiring lines from the source-side n⁺ type portion 138S and thegate-side n⁺ type portion 138G of each of the diode units 140 in adirection parallel with the substrate 122, which may save space.

The cut-out region 107 for providing insulation between the sourceterminal 102 and the gate terminal 103 may be formed into a zigzagshape. Thus, the source extension portions 114 and the gate extensionportions 115 required for making contact with each of the diode units140 are alternately arranged so as to mesh with each other in acomb-like manner. Accordingly, the diode units 140 connected between thesource extension portions 114 and the gate extension portions 115adjoining the source extension portions 114 can be formed in pluralnumbers along the circumferential direction of the gate terminal 103.Therefore, the diode units 140 having a high protection tolerance can beformed in plural numbers by appropriately positioning the sourceextension portions 114 and the gate extension portions 115, andadjusting the length L₁ of the respective diode units 140 interposedtherebetween to become equal to 10 μm to 50 μm. Further, the space leftbetween the source terminal 102 and the gate terminal 103 can beeffectively used for a contact with the double-ended zener diode 111.

As shown in FIG. 14, the double-ended zener diodes 111 are formed overthe entire circumference of the diode region 109 so as to surround theactive region 110. Accordingly, the total area of the double-ended zenerdiodes 11 lean increase as compared with the case where the double-endedzener diodes 111 are locally arranged along the circumferentialdirection of the diode region 109. As a result, the protection tolerance(the electrostatic breakdown voltage) of the double-ended zener diodes111 can increase. Like the double-ended zener diode 111′ shown in FIG.14 (the hatched portion in FIG. 14), the double-ended zener diode 111may be locally formed in, e.g., the portion surrounding the gate pad105.

<Modified Example of Double-Ended Zener Diode>

FIG. 27 illustrates a modified example of the double-ended zener diode151. FIG. 28 illustrates a first modified example of the source terminal102 and the gate terminal 103. FIG. 29 illustrates a second modifiedexample of the source terminal 102 and the gate terminal 103.

In FIGS. 14 and 15, as one example of the double-ended zener diodes, thedouble-ended zener diodes 111 formed in a ring-shaped pattern bycontinuously forming the diode units 140 along the circumferentialdirection of the diode region 109 (the gate terminal 103) are adopted.Alternatively, like the double-ended zener diodes 151 as second diodesshown in FIG. 27, the respective diode units 152 may be independentlyformed to extend across the diode region 109 (the gate terminal 103).

More specifically, in the double-ended zener diodes 151 shown in FIG.27, unlike the diode trenches 135 shown in FIG. 15, which collectivelyaccommodate the diode units 140, the diode trenches 135 are formed toextend from the source extension portions 114 to the gate terminal 103(not to the gate extension portions 115 but to the body portion of thegate terminal 103) along the extending direction of the source extensionportions 114. Thus, each of the diode units 152 extends from the sourceextension portions 114 to the gate terminal 103 along the extendingdirection of the source extension portions 114 so that the source-siden⁺ type portion 138S and the gate-side n⁺ type portion 138G are arrangeddirectly below the source extension portions 114 and the gate terminal103 (not directly below the gate extension portions 115 but directlybelow the body portion), respectively.

The respective diode units 152 (the diode trenches 135) are formed in aone-to-one correspondence with the source extension portions 114 andarranged so that the double-ended zener diodes 151 as a whole surroundthe active region 110. The diode units 152 need not be formed in all ofthe source extension portions 114 and may be locally formed between someof the source extension portions 114 and the gate terminal 103.

With this modified example, the diode units 152 can be formed atsuitable locations of the diode region 109 in a suitable number.

In the case of providing the double-ended zener diodes 151 shown in FIG.27, the gate extension wiring line 121 may be elongated to reach thegate terminal 103 (the body portion of the gate terminal 103), and theMOS gate contact 119 may be formed at the distal end of the gateextension wiring line 121, as shown in FIG. 28. With this configuration,the gate extension portions 115 can be omitted.

In addition, for example, as shown in FIG. 29, the entire peripheraledge portion of the source terminal 102 may extend from the areadirectly above the unit cells 112 instead of locally forming the sourceextension portions 114. In this case, the source contacts 116 may beformed in the extended area. With this configuration, the sourceextension portions 114 can be omitted.

While an embodiment of the reference example has been described above,the reference example can be embodied in other forms.

In the embodiment described above, the trench-gate-type MOSFET 113 isformed in the active region 110 of the semiconductor device 101.Alternatively, as in a semiconductor device 153 shown in FIGS. 30A and30B, a trench-gate-type IGBT (Insulated Gate Bipolar transistor) 154 maybe formed in the active region 110.

The semiconductor device 153 shown in FIGS. 30A and 30B includes asubstrate 155 made of p⁺ type silicon instead of the n⁺ type substrate122. An emitter region 156 of the IGBT 154 corresponds to the sourceregion 127 of the MOSFET 113. A base region 157 corresponds to thechannel region 128. A contact region 158 corresponds to the drift region130. An emitter-side n⁺ type portion 138E corresponds to the source-siden⁺ type portion 138S. An emitter terminal 159 corresponds to the sourceterminal 102. A collector terminal 160 corresponds to the drain terminal147.

In the IGBT 154, a double-ended zener diode 1 is connected between theemitter terminal 159 and the gate terminal 103 (between the emitter andthe gate). Accordingly, the same advantageous effects as in thesemiconductor device 101 of the foregoing embodiment can be obtained.

The opposite n⁺ type portions of the diode units 140 and 152 from thegate-side n⁺ type portion 138G may be connected to the drain terminal147 and the contact terminal 160, respectively, instead of beingconnected to the source terminal 102 and the emitter terminal 159,respectively. With this configuration, the double-ended zener diodes 111and 151 can be connected between the drain and the gate and between thecollector and the gate, respectively.

In the case where the n⁺ type portion 138 is connected to the drainterminal 147, as shown in FIGS. 31, 32A and 32B, a drain contact 163 isformed by forming a contact hole 161 extending from the bottom surfaceof the diode trench 135 to the substrate 122 through the epitaxial layer124, forming an insulation film 162 on the inner surface of the contacthole 161 and filling a metallic material into the inside of theinsulation film 162. The n⁺ type portion 138 (a drain-side n⁺ typeportion 138D) and the drain terminal 147 can be connected to each otherthrough the drain contact 163.

Accordingly, as shown in FIG. 33, the double-ended zener diodes 111(Z_(D)) can be connected between the source terminal 102 (S) and thegate terminal 103 (G) (between the source and the gate) and between thedrain terminal 147 (D) and the gate terminal 103 (G) (between the drainand the gate) in the semiconductor device 101. Reference symbol D₁designates a substrate diode which is connected between the sourceterminal 102 (S) and the drain terminal 147 (D). In this configuration,however, the double-ended zener diode 111 may be omitted by omitting thesource contact 116.

In the case where the n⁺ type portion 138 is connected to a collectorterminal 160, as shown in FIGS. 34A and 34B, a collector contact 166 isformed by forming a contact hole 164 extending from the bottom surfaceof the diode trench 135 to the substrate 155 through the epitaxial layer124, forming an insulation film 165 on the inner surface of the contacthole 164 and filling a metallic material into the inside of theinsulation film 165. The n⁺ type portion 138 (a collector-side n⁺ typeportion 138C) and the collector terminal 160 can be connected to eachother through the collector contact 166.

In the foregoing embodiment, the respective diode units 140 and 152 andthe respective terminals are connected to one another by the columnarplug contacts extending through the inter-layer insulation film 141 inthe thickness direction. Alternatively, rewiring lines may be formed inthe respective layers by forming a multi-layer wiring structure betweenthe respective diode units 140 and 152 and the respective terminals. Therespective diode units 140 and 152 and the respective terminals may beconnected to one another using the multiple layers of the rewiringlines.

Insofar as the gate terminal 103 can be connected to the gate-side n⁺type portions 138G of the respective diode units 140 and 152, the gateterminal 103 does not need to surround the source terminal 102 as shownin FIG. 14. For example, if the diode region 109 is locally formedaround the source terminal 102 when seen in a plan view, the gateterminal 103 may be formed to surround the diode region 109.

The gate trenches 126 and the diode trenches 135 do not need to beformed in the same step as shown in FIGS. 20A and 20B. The gate trenches126 and the diode trenches 135 may be formed by independently performinga step of forming the gate trenches 126 and a step of forming the diodetrenches 135.

Further, a configuration in which the conductivity types of therespective semiconductor portions of the semiconductor devices 101 and153 are inverted may be employed. For example, in the semiconductordevices 101 and 153, the p type portions may be n type portions and then type portions may be p type portions.

In the semiconductor devices 101 and 153, the layer making up theepitaxial layer 124 is not limited to the epitaxial layer 124 made of Sibut may be a layer made of, e.g., a wide-band-gap semiconductor such asSiC, GaN or diamond.

<Features Grasped from Disclosure of Embodiments of Reference Example>

The following features (1) through (20) can be grasped, for example,from the disclosure of the embodiments of the reference example.

(1) A semiconductor device, including: a semiconductor layer having aplurality of impurity regions forming a transistor; a gate trench formedin the semiconductor layer; a gate electrode buried in the gate trenchand having an upper surface flush with a front surface of thesemiconductor layer; a diode trench formed in the semiconductor layer;and a double-ended zener diode buried in the diode trench and having anupper surface flush with the front surface of the semiconductor layer,wherein the double-ended zener diode includes a plurality of pnjunctions in which n type portions and p type portions are alternatelyarranged along a direction orthogonal to a thickness direction of thediode trench.

(2) The semiconductor device of (1), further including: an inter-layerinsulation film formed on the semiconductor layer, a first terminalformed on the inter-layer insulation film and connected to the gateelectrode through the inter-layer insulation film; and a second terminalformed on the inter-layer insulation film and connected to the impurityregions of the semiconductor layer through the inter-layer insulationfilm, wherein the first terminal and the second terminal arerespectively connected through the inter-layer insulation film to the ntype portions or the p type portions spaced apart from each other.

(3) The semiconductor device of (2), wherein the double-ended zenerdiode is formed to extend from the first terminal to the second terminalwhen seen in a plan view such that the n type portions or the p typeportions spaced apart from each other are positioned directly below thefirst terminal and the second terminal, respectively.

(4) The semiconductor device of (2) or (3), wherein the first terminalincludes a ring-shaped terminal surrounding the second terminal.

(5) The semiconductor device of (4), wherein the ring-shaped terminalincludes first extension portions protruding toward the second terminal,the second terminal surrounded by the ring-shaped terminal includessecond extension portions protruding toward the ring-shaped terminal,and the double-ended zener diode includes a first diode in which the pnjunctions are arranged along a circumferential direction of thering-shaped terminal such that the n type portions or the p typeportions spaced apart from each other are positioned directly below thefirst extension portions and the second extension portions,respectively.

(6) The semiconductor device of (5), wherein the first diode is formedinto a ring shape so as to surround the second terminal.

(7) The semiconductor device of (5) or (6), wherein the first extensionportions and the second extension portions are alternately arrangedalong the circumferential direction of the ring-shaped terminal.

(8) The semiconductor device of (4), wherein the double-ended zenerdiode includes a second diode in which the pn junctions are arranged toextend across the ring-shaped terminal such that the n type portions orthe p type portions spaced apart from each other are positioned directlybelow the ring-shaped terminal and the second terminal, respectively.

(9) The semiconductor device of any one of (1) through (8), wherein theimpurity regions include a MOS structure forming a trench-gate-typeMOSFET (Metal Oxide Semiconductor Field Effect Transistor) incooperation with the gate electrode, the MOS structure including: afirst-conductivity-type source region exposed on the front surface ofthe semiconductor layer and configured to form a portion of a sidesurface of the gate trench; a second-conductivity-type channel regionformed at a side of a rear surface of the semiconductor layer withrespect to the source region so as to make contact with the sourceregion and configured to form a portion of the side surface of the gatetrench; and a first-conductivity-type drain region formed at the side ofthe rear surface of the semiconductor layer with respect to the channelregion so as to make contact with the channel region and configured toform a bottom surface of the gate trench.

(10) The semiconductor device of (9) associated with (2) through (8),wherein the second terminal includes a source terminal connected to thesource region.

(11) The semiconductor device of (9) associated with (2) through (8),wherein the second terminal includes a drain terminal connected to thedrain region.

(12) The semiconductor device of any one of (1) through (8), wherein theimpurity regions include an IGBT structure forming a trench-gate-typeIGBT (Insulated Gate Bipolar Transistor) in cooperation with the gateelectrode, the IGBT structure including: a first-conductivity-typeemitter region configured to form a portion of a side surface of thegate trench; a second-conductivity-type base region formed at a side ofa rear surface of the semiconductor layer with respect to the emitterregion so as to make contact with the emitter region and configured toform a bottom surface of the gate trench; and a first-conductivity-typecollector region formed at the side of the rear surface of thesemiconductor layer with respect to the base region so as to makecontact with the channel region.

(13) The semiconductor device of (12) associated with (2) through (8),wherein the second terminal includes an emitter terminal connected tothe emitter region.

(14) The semiconductor device of (12) associated with (2) through (8),wherein the second terminal includes a collector terminal connected tothe collector region.

(15) The semiconductor device of any one of (1) through (14), whereinthe width of the diode trench is 1.2 to 5.0 times as large as the widthof the gate trench.

(16) The semiconductor device of any one of (1) through (15), whereinthe double-ended zener diode is made of polysilicon.

(17) A manufacturing method of a semiconductor device, including:forming a transistor having a plurality of impurity regions on asemiconductor layer; forming a gate trench by etching the semiconductorlayer from the front surface thereof; forming a diode trench by etchingthe semiconductor layer from the front surface thereof; depositing an ntype or a p type semiconductor material on the semiconductor layer so asto fill the gate trench and the diode trench; simultaneously forming agate electrode having an upper surface flush with the front surface ofthe semiconductor layer and a buried layer having an upper surface flushwith the front surface of the semiconductor layer by etching back thedeposited semiconductor material; and forming a double-ended zener diodewithin the diode trench by selectively implanting n type or p typeimpurities differing in conductivity type from the buried layer into theburied layer and forming a plurality of pn junctions in which n typeportions and p type portions are alternately arranged along a directionorthogonal to a thickness direction of the diode trench.

(18) The manufacturing method of a semiconductor device of (17), furtherincluding: forming an inter-layer insulation film on the semiconductorlayer; forming a first terminal on the inter-layer insulation film, thefirst terminal being connected through the inter-layer insulation filmto the n type portions or the p type portions of one of the gateelectrode and the double-ended zener diode; and forming a secondterminal on the inter-layer insulation film, the second terminal beingconnected through the inter-layer insulation film to the n type portionsor the p type portions spaced apart from the impurity regions of thesemiconductor layer and spaced apart from the n type portions or the ptype portions connected to the first terminal, and the n type portionsor the p type portions connected to the second terminal having the sameconductivity type as the conductivity type of the n type portions or thep type portions connected to the first terminal.

(19) The manufacturing method of a semiconductor device of (18), whereinsaid forming the first terminal and the second terminal includessimultaneously forming the first terminal and the second terminal bydepositing a metallic material on the entire surface of the inter-layerinsulation film and then patterning the metallic material into aspecified shape.

(20) The manufacturing method of any one of (17) through (19), whereinsaid forming the gate trench and said forming the diode trench areperformed by the same etching.

<Advantageous Effects of Features Grasped Above>

With the semiconductor device of (1), the upper surface of the gateelectrode and the upper surface of the double-ended zener diode areflush with the front surface of the semiconductor layer. Accordingly,the gate electrode and the double-ended zener diode (the buried layerforming the body of the double-ended zener diode) can be formedsimultaneously by etching the semiconductor layer from the front surfacethereof to form the gate trench and the diode trench, depositing the ntype or the p type semiconductor material so as to fill the gate trenchand the diode trench, and then etching back the deposited semiconductormaterial.

Therefore, the manufacturing process of the semiconductor device can besimplified and the manufacturing cost can be reduced as compared with aconventional method. In the conventional method, a step of forming agate electrode so that the upper surface thereof becomes flush with thefront surface of an epitaxial layer and a step of forming a zener diodeso as to protrude from a diode trench toward the front surface of theepitaxial layer are performed independently from each other.

Further, in the conventional method, the zener diode is shaped toprotrude from the diode trench. For this reason, after depositingpolysilicon so as to fill the diode trench, it is necessary to perform astep of patterning the polysilicon using a mask, which may deteriorateefficiency.

Further, in the conventional method, the zener diode protruding from thediode trench is left as a step difference with respect to the frontsurface of the epitaxial layer. As a result, the step difference (aheight deviation) is generated in the inter-layer insulation film formedso as to cover the front surface of the epitaxial layer. For thisreason, it is likely that a focus abnormality may occur when aphotoresist is exposed to form contact holes in the inter-layerinsulation film. In addition, it is likely that the inter-layerinsulation film will have a discontinuous portion at the step differenceand a wiring metal will easily come into the discontinuous portion.Therefore, it is likely that the wiring metal is left as a residue(metal residue) when the source electrode or the like is formed bypatterning the wiring metal, and thus wiring lines may short-circuit.

In contrast, with the disclosure of the reference example, the uppersurface of the gate electrode and the upper surface of the double-endedzener diode can be made flush with the front surface of thesemiconductor layer. Accordingly, the focus abnormality problem and themetal residue problem inherent in the conventional method can be solved.As a result, the quality and performance of the semiconductor device canbe improved.

The semiconductor device of (1) can be manufactured by, e.g., themanufacturing method of a semiconductor device of (17).

If the semiconductor device of (1) includes, as recited in (2), aninter-layer insulation film formed on the semiconductor layer, a firstterminal formed on the inter-layer insulation film and connected to thegate electrode through the inter-layer insulation film, and a secondterminal formed on the inter-layer insulation film and connected to theimpurity regions of the semiconductor layer through the inter-layerinsulation film, the first terminal and the second terminal can beconnected through the inter-layer insulation film to the n type portionsor the p type portions spaced apart from each other.

In other words, the first terminal and the second terminal can beconnected to the mutually spaced-apart n type portions of thedouble-ended zener diode or can be connected to the mutuallyspaced-apart p type portions of the double-ended zener diode. With thisconfiguration, the double-ended zener diode can be connected between thegate electrode and a specific one of the impurity regions (between thegate and the impurity regions).

As recited in (18), the semiconductor device can be manufactured by:forming an inter-layer insulation film on the semiconductor layer,forming a first terminal on the inter-layer insulation film, the firstterminal being connected through the inter-layer insulation film to then type portions or the p type portions of one of the gate electrode andthe double-ended zener diode; and forming a second terminal on theinter-layer insulation film, the second terminal being connected throughthe inter-layer insulation film to the n type portions or the p typeportions spaced apart from the impurity regions of the semiconductorlayer and spaced apart from the n type portions or the p type portionsconnected to the first terminal, and the n type portions or the p typeportions connected to the second terminal having the same conductivitytype as the conductivity type of the n type portions or the p typeportions connected to the first terminal.

In this case, as recited in (19), said forming the first terminal andthe second terminal may include simultaneously forming the firstterminal and the second terminal by depositing a metallic material onthe entire surface of the inter-layer insulation film and thenpatterning the metallic material into a specified shape.

With this method, the first terminal and the second terminal can beformed in the same step. Therefore, the semiconductor device can be moreeasily and cost-effectively manufactured.

If said forming the gate trench and said forming the diode trench areperformed by the same etching, as recited in (20), the semiconductordevice can be more easily and more cost-effectively manufactured

As recited in (3), the double-ended zener diode may be formed to extendfrom the first terminal to the second terminal when seen in a plan viewsuch that the n type portions or the p type portions spaced apart fromeach other are positioned directly below the first terminal and thesecond terminal, respectively.

With this configuration, the first terminal and the second terminal canbe connected to the n type portions or the p type portions using theplugs extending directly downward from the first terminal and the secondterminal through the inter-layer insulation film. Therefore, it is notnecessary to draw the wiring lines from the n type portions or the ptype portions in a horizontal direction, which may save space.

As recited in (4), the first terminal may include a ring-shaped terminalsurrounding the second terminal.

As recited in (5), if the ring-shaped terminal includes first extensionportions protruding toward the second terminal and the second terminalsurrounded by the ring-shaped terminal includes second extensionportions protruding toward the ring-shaped terminal, the double-endedzener diode may include a first diode in which the pn junctions arearranged along a circumferential direction of the ring-shaped terminalsuch that the n type portions or the p type portions spaced apart fromeach other are positioned directly below the first extension portionsand the second extension portions, respectively.

With this configuration, the first extension portions and the secondextension portions are formed in the space provided to insulate thefirst terminal and the second terminal. Thus the remaining space can beeffectively used for making connection to the double-ended zener diode.

As recited in (6), the first diode may be formed into a ring shape so asto surround the second terminal.

With this configuration, the area of the first diode can increase ascompared with the case where the first diode is locally arranged alongthe circumferential direction of the ring-shaped terminal, which mayincrease the protection tolerance (the electrostatic breakdown voltage)of the first diode.

As recited in (7), the first extension portions and the second extensionportions may be alternately arranged along the circumferential directionof the ring-shaped terminal.

With this configuration, the units of the double-ended zener diodesconnected between the first extension portions and the second extensionportions adjoining the first extension portions can be formed in pluralnumbers along the circumferential direction of the ring-shaped terminal.Therefore, the double-ended zener diodes having a high protectiontolerance can be formed in plural numbers by appropriately positioningthe first extension portions and the second extension portions, andadjusting the length of the pn repetition units of the double-endedzener diodes interposed therebetween.

If the first terminal includes the ring-shaped terminal surrounding thesecond terminal, as recited in (8), the double-ended zener diode mayinclude a second diode in which the pn junctions are arranged to extendacross the ring-shaped terminal such that the n type portions or the ptype portions spaced apart from each other are positioned directly belowthe ring-shaped terminal and the second terminal, respectively.

With this configuration, the second diode does not need to be formedalong the ring-shaped terminal. Therefore, the double-ended zener diodescan be formed at suitable locations in a suitable number.

As recited in (9), the impurity regions may include a MOS structureforming a trench-gate-type MOSFET (Metal Oxide Semiconductor FieldEffect Transistor) in cooperation with the gate electrode. In that case,the MOS structure may include: a first-conductivity-type source regionexposed on the front surface of the semiconductor layer and configuredto form a portion of the side surface of the gate trench; asecond-conductivity-type channel region formed at a side of a rearsurface of the semiconductor layer with respect to the source region soas to make contact with the source region and configured to form aportion of the side surface of the gate trench; and afirst-conductivity-type drain region formed at the side of the rearsurface of the semiconductor layer with respect to the channel region soas to make contact with the channel region and configured to form abottom surface of the gate trench.

Thus the double-ended zener diode can be used as a gate-protecting diodeof the trench-gate-type MOSFET. In this case, as recited in (10), thesource terminal (the first terminal) connected to the source region maybe connected to the double-ended zener diode, thereby connecting thedouble-ended zener diode between the gate electrode and the sourceregion (between the gate and the source). Alternatively, as recited in(11), the drain terminal (the second terminal) connected to the drainregion may be connected to the double-ended zener diode, therebyconnecting the double-ended zener diode between the gate electrode andthe drain region (between the gate and the drain).

As recited in (12), the impurity regions may include an IGBT structureforming a trench-gate-type IGBT (Insulated Gate Bipolar Transistor) incooperation with the gate electrode. In this case, the IGBT structuremay include: a first-conductivity-type emitter region configured to forma portion of the side surface of the gate trench; asecond-conductivity-type base region formed at a side of a rear surfaceof the semiconductor layer with respect to the emitter region so as tomake contact with the emitter region and configured to form a bottomsurface of the gate trench; and a first-conductivity-type collectorregion formed at the side of the rear surface of the semiconductor layerwith respect to the base region so as to make contact with the channelregion.

Thus, the double-ended zener diode can be used as a gate-protectingdiode of the trench-gate-type IGBT. In this case, as recited in (13),the emitter terminal (the first terminal) connected to the emitterregion may be connected to the double-ended zener diode, therebyconnecting the double-ended zener diode between the gate electrode andthe emitter region (between the gate and the emitter). Alternatively, asrecited in (14), the collector terminal (the second terminal) connectedto the collector region may be connected to the double-ended zenerdiode, thereby connecting the double-ended zener diode between the gateelectrode and the collector region (between the gate and the collector).

As recited in (15), the width of the diode trench may be 1.2 to 5.0times as large as the width of the gate trench.

With this configuration, variations of the width of the diode trench andthe width of the gate trench can be kept within a specified range.Therefore, the occurrence of dishing in the gate electrode and thedouble-ended zener diode (the buried layer), e.g., when etching back thesemiconductor material, can be prevented.

As recited in (16), the double-ended zener diode may be made ofpolysilicon.

By using polysilicon as an electrode material, the semiconductor devicecan be more easily and cost-effectively manufactured.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosures. Indeed, the novel devices and methodsdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe embodiments described herein may be made without departing from thespirit of the disclosures. The accompanying claims and their equivalentsare intended to cover such forms or modifications as would fall withinthe scope and spirit of the disclosures.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor layer having a plurality of impurity regions forming atransistor; a gate trench formed in the semiconductor layer; a gateelectrode buried in the gate trench and having an upper surface flushwith a front surface of the semiconductor layer; a diode trench formedin the semiconductor layer; and a double-ended zener diode buried in thediode trench and having an upper surface flush with the front surface ofthe semiconductor layer, the double-ended zener diode including aplurality of pn junctions in which n type portions and p type portionsare alternately arranged along a direction orthogonal to a thicknessdirection of the diode trench.
 2. The semiconductor of claim 1, furthercomprising: an inter-layer insulation film formed on the semiconductorlayer; a first terminal formed on the inter-layer insulation film andconnected to the gate electrode through the inter-layer insulation film;and a second terminal formed on the inter-layer insulation film andconnected to the impurity regions of the semiconductor layer through theinter-layer insulation film, and wherein the first terminal and thesecond terminal are respectively connected through the inter-layerinsulation film to the n type portions or the p type portions spacedapart from each other.
 3. The semiconductor device of claim 2, whereinthe double-ended zener diode is formed to extend from the first terminalto the second terminal when seen in a plan view such that the n typeportions or the p type portions spaced apart from each other arepositioned directly below the first terminal and the second terminal,respectively.
 4. The semiconductor device of claim 2, wherein the firstterminal includes a ring-shaped terminal surrounding the secondterminal.
 5. The semiconductor device of claim 4, wherein thering-shaped terminal includes first extension portions protruding towardthe second terminal, wherein the second terminal surrounded by thering-shaped terminal includes second extension portions protrudingtoward the ring-shaped terminal, and wherein the double-ended zenerdiode includes a first diode in which the pn junctions are arrangedalong a circumferential direction of the ring-shaped terminal such thatthe n type portions or the p type portions spaced apart from each otherare positioned directly below the first extension portions and thesecond extension portions, respectively.
 6. The semiconductor device ofclaim 5, wherein the first diode is formed into a ring shape so as tosurround the second terminal.
 7. The semiconductor device of claim 5,wherein the first extension portions and the second extension portionsare alternately arranged along the circumferential direction of thering-shaped terminal.
 8. The semiconductor device of claim 4, whereinthe double-ended zener diode includes a second diode in which the pnjunctions are arranged to extend across the ring-shaped terminal suchthat the n type portions or the p type portions spaced apart from eachother are positioned directly below the ring-shaped terminal and thesecond terminal, respectively.
 9. The semiconductor device of claim 2,wherein the impurity regions include a MOS structure forming atrench-gate-type MOSFET (Metal Oxide Semiconductor Field EffectTransistor) in cooperation with the gate electrode, the MOS structureincluding: a first-conductivity-type source region exposed on the frontsurface of the semiconductor layer and configured to form a portion of aside surface of the gate trench; a second-conductivity-type channelregion formed at a side of a rear surface of the semiconductor layerwith respect to the source region so as to make contact with the sourceregion and configured to form a portion of the side surface of the gatetrench; and a first-conductivity-type drain region formed at the side ofthe rear surface of the semiconductor layer with respect to the channelregion so as to make contact with the channel region and configured toform a bottom surface of the gate trench.
 10. The semiconductor deviceof claim 9, wherein the second terminal includes a source terminalconnected to the source region.
 11. The semiconductor device of claim 9,wherein the second terminal includes a drain terminal connected to thedrain region.
 12. The semiconductor device of claim 2, wherein theimpurity regions include an IGBT structure forming a trench-gate-typeIGBT (Insulated Gate Bipolar Transistor) in cooperation with the gateelectrode, the IGBT structure including: a first-conductivity-typeemitter region configured to form a portion of a side surface of thegate trench; a second-conductivity-type base region formed at a side ofa rear surface of the semiconductor layer with respect to the emitterregion so as to make contact with the emitter region and configured toform a bottom surface of the gate trench; and a first-conductivity-typecollector region formed at the side of the rear surface of thesemiconductor layer with respect to the base region so as to makecontact with the channel region.
 13. The semiconductor device of claim12, wherein the second terminal includes an emitter terminal connectedto the emitter region.
 14. The semiconductor device of claim 12, whereinthe second terminal includes a collector terminal connected to thecollector region.
 15. The semiconductor device of claim 1, a width ofthe diode trench is 1.2 to 5.0 times as large as a width of the gatetrench.
 16. The semiconductor device of claim 1, wherein thedouble-ended zener diode is made of polysilicon.
 17. A manufacturingmethod of a semiconductor device, comprising: forming a transistorhaving a plurality of impurity regions on a semiconductor layer; forminga gate trench by etching the semiconductor layer from a front surfacethereof; forming a diode trench by etching the semiconductor layer fromthe front surface thereof; depositing an n type or a p typesemiconductor material on the semiconductor layer so as to fill the gatetrench and the diode trench; simultaneously forming a gate electrodehaving an upper surface flush with the front surface of thesemiconductor layer and a buried layer having an upper surface flushwith the front surface of the semiconductor layer by etching back thedeposited semiconductor material; and forming a double-ended zener diodewithin the diode trench by selectively implanting n type or p typeimpurities differing in conductivity type from the buried layer into theburied layer and forming a plurality of pn junctions in which n typeportions and p type portions are alternately arranged along a directionorthogonal to a thickness direction of the diode trench.
 18. Themanufacturing method of claim 17, further comprising: forming aninter-layer insulation film on the semiconductor layer; forming a firstterminal on the inter-layer insulation film, the first terminal beingconnected through the inter-layer insulation film to the n type portionsor the p type portions of one of the gate electrode and the double-endedzener diode; and forming a second terminal on the inter-layer insulationfilm, the second terminal being connected through the inter-layerinsulation film to the impurity regions of the semiconductor layer andthe n type portions or the p type portions spaced apart from the n typeportions or the p type portions connected to the first terminal, and then type portions or the p type portions connected to the second terminalhaving the same conductivity type as the conductivity type of the n typeportions or the p type portions connected to the first terminal.
 19. Themanufacturing method of claim 18, wherein the first terminal and thesecond terminal are simultaneously formed by depositing a metallicmaterial on the entire surface of the inter-layer insulation film andthen patterning the metallic material into a specified shape.
 20. Themanufacturing method of claim 17, wherein forming the gate trench andforming the diode trench are performed by the same etching.